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212 


Brovni— 


^Handbook  of 
~caFburetior 


Southern  Branch 
of  the 

University  of  California 

Los  Angeles 

Form  L  1 

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212 
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UNIVERSITY  OF  CALIFORNIA  LIBRARY 

Los  Angeles 
This  book  is  DUE  on  the  last  date  stamped  below. 


Form  L9-32m-8,'58(5876s4) 444 


HANDBOOK 


OF 


CARBURETION 


BY 

ARTHUR  BENJ.   BROWNE 

Consulting  Engineer 

Member  Society  of  Automobile  Engineers 
Member  American  Institute  of  Mining  Engineers 


FIRST  EDITION 
FIRST  THOUSAND 


NEW   YORK 
JOHN  WILEY  &  SONS,  INC. 

LONDON:    CHAPMAN   &   HALL,    LIMITED 
1916 


51936 


Copyright,  1915,  by 
ARTHUR  B.    BROWNE 


PUBLISHERS  PRINTING  COMPANY 
207-217  West  Twenty-fifth  Street.  New  York 


TL 
£/  *. 

"3 -si 


FOREWORD 

CONSENSUS  of  public  opinion,  both  technical  and  lay,  would 
undoubtedly  be  singularly  unanimous  in  welcoming  the  complete 
abolition  of  the  carbureter.  No  part  of  a  motor-vehicle  is  less 
understood  or  more  abused,  in  thought  and  deed.  No  other 
part  of  the  entire  mechanism  .of  the  car  is  subjected  to  the  in- 
^  dignities  that  are  heaped  upon  the  carbureter. 

This  condition  will  continue  to  exist  until  the  genius  which  has 
"    already  made  such  colossal  strides  in  automobile    engineering 
turns  its  serious  attention  to  an  understanding  of  the  fundamental 
laws  governing  carburetion. 

The  development  of  any  branch  of  science  depends  largely 

*l      on  the  recognition  of  fundamental  principles.    These  principles 

\     may  be  as  accurately  determinative  as  are  those  expressed  in 

Ohm's  law,  or  as  purely  theoretical  as  is  Dalton's  Atomic  Theory, 

but  the  science  of  chemistry  surely  owes  no  less  to  Dalton  than 

the  electrical  field  owes  to  Ohm. 

In  the  science  of  carburetion  it  is  difficult,  perhaps  impossible, 

V    to  secure  practical  measurements  as  definite  as  those  from  which 

vj     Ohm  deduced  his  law,  but  the  application  of  natural  laws  to 

^     problems  of  carburetion  is  so  far  less  an  excursion  into  the  realms 

of  pure  theory  than  that  by  which  Dalton  revolutionized  science, 

that  its  recognition  and  universal  acceptance  by  practical  men 

seems  overlong  delayed.     Still,  the  lack  of  uniformity  exhibited 

by   the  great  and  ever-increasing  variety  of  carbureters  on 

the  market  proves  that,  as  yet,  no  comprehensive  principle  of 

automatic  regulation  of  the  gas  to  air  ratio  has  been  generally 

recognized. 

The  simplest  form  of  carbureting  device  consists  of  a  fuel 
jet  introduced  into  the  moving  air  column  within  the  intake  pipe. 
If  the  velocity  of  the  fuel  flow  were  directly  proportional  to  the 
velocity  of  the  air  flow,  the  mixture  from  such  a  device  would  be 

iii 


IV  FOREWORD 

of  constant  composition  under  all  conditions  and  the  principal 
problem  of  carburetion  would  be  resolved  at  once  to  its  simplest 
terms.  Unfortunately,  the  relation  between  the  air  and  fuel 
velocities  is  not  a  direct  proportion,  but,  as  will  be  demonstrated, 
it  is  none  the  less  definite.  Once  recognized,  its  application  to 
practical  carburetion  not  only  eliminates  the  necessity  for  most 
of  the  mechanical  complications  now  in  use,  but  it  explains 
clearly  the  errors  which  are  introduced  by  their  use. 


TABLE   OF   CONTENTS 

CHAPTER   I 

THEORY  OF   CARBURETION 

PAGE 

BASIC  PRINCIPLES i 

EFFECT  OF  TEMPERATURE 2 

THE  LAW  APPLIED  TO  TYPES 4 

The  simple  carbureter 4 

The  compensating  carbureter 5 

The  multiple  jet  carbureter 8 

The  variable  fuel  orifice 9 

The  constant  vacuum  principle 10 

The  compensating  nozzle 1 1 

Compensation  by  velocities 13 

Relation  of  velocity  to  vacuum 15 

Variable  mixtures 16 

Constant  mixture,  advantages  of 17 

Velocity  the  only  constant 17 

Loss  of  volumetric  efficiency 1 8 

CHAPTER   II 

THE   INTAKE   MANIFOLD 

Functions  of  the  manifold 20 

Carbureter  product  not  a  gas         ...            20 

Necessity  for  proper  velocities       .                   21 

Deposition  not  condensation 22 

Surging 23 

Causes  of  hard  starting 24 

Carburetion  within  the  manifold 24 

Atomization,  effect  of 26 

Area  of  the  manifold 26 

Condition  of  smoothness 26 

Length  of  manifold,  effect  of 27 

Diffusion 27 

Types  of  manifolds 28 

Effect  of  bend 31 

Conclusions 33 


VI  TABLE    OF    CONTENTS 

CHAPTER   III 

CARBURETER  TESTING 

PAGE 

ON  THE  BLOCK :     .     .  35 

Usual  methods       .      . 35 

Maximum  horse-power 35 

Fallacy  of  set  throttle  tests 36 

Testing  with  fixed  load       .      .  • 36 

Flexibility 38 

Practical  results 38 

Automatic  apparatus 39 

Difficulty  in  comparing  results 39 

THE   ACCELEROMETER   .        ...        .        .        .        .        .        .        .        .        .        ...  4! 

Principle  of  operation 41 

Principle  of  compensation 42 

Acceleration  up-grade 43 

Acceleration  down-grade        . 44 

Retardation  up-grade 44 

Retardation  down-grade 44 

Determination  of  resistance 45 

Method  of  reading : .  45 

Total  resistance 45 

Measuring  engine  friction 45 

Measuring  transmission  friction         46 

Locating  mechanical  defects 47 

Determination  of  draw-bar  pull         47 

Determination  of  B.  H.  P 48 

Determination  of  I.  H.  P 48 

Basis  for  comparing  performances 48 

Determination  of  thermal  efficiency 49 

Accuracy 50 

Levelling 51 

Effect  of  wind 51 

CHAPTER   IV 

THE  PRACTICAL   TESTING  OF   MOTOR-VEHICLES 

Road  testing 52 

Proposed  test  of  performance 52 

Rolling  resistance 54 

Description  of  apparatus,  Yale  University 55 

Method  of  testing 58 

Description  of  runs 58 

Diagram  of  results 59 

Report  form 61 

Acceleration  and  hill-climbing  ability 63 


TABLE    OF    CONTENTS  vil 

PAGE 

Speed  range 64 

Fuel  consumption 65 

Check  of  speed  limit 66 

Applicability  to  road  conditions 68 

Disclosure  of  characteristics 70 

Performance  test  as  a  basis  for  detailed  investigation 74 

Draw-bar  pull  vs.  horse-power 75 


CHAPTER   V 

DIRECT   DETERMINATION  OF   CARBURETER  ACTION 

THE  ANEMOMETER 76 

ORIFICE  IN  THIN  PLATE 76 

Durley's  formula  of  flow 77 

Thickness  of  the  plate 77 

Necessary  conditions 77 

Coefficients  of  flow 78 

Apparatus  for  carbureter  measurements 78 

The  rubber  diaphragm 79 

Proper  orifice  diameters 79 

Objection  to  the  method 80 

THE  VENTURI  METER 80 

Principles  involved 80 

Calibration 81 

Barometric  and  temperature  correction 8 1 

Application  to  carbureter  measurements 81 

CHAPTER   VI 

THE   CHEMISTRY  OF   CARBURETION 

INTRODUCTION 83 

AVAILABILITY  OF  EXHAUST  GAS  ANALYSIS 83 

COMBUSTION 84 

Definition 84 

Reactions 84 

CHEMICAL  COMPOSITION  OF  AIR 85 

Elements 86 

Temperature  correction .86 

Compounds 87 

Final  air/gas  ratios 87 

Loss  FROM  INCOMPLETE  COMBUSTION 87 

Thermal  losses -87 

Watson's  diagram  of 88 

Dangerous  characteristics  of  the  exhaust 88 

Economic  character  of  the  exhaust 89 


viii  TABLE    OF    CONTENTS 

PAGE 

DETERMINATION  OF  AIR/GAS  RATIOS  BY  ANALYSIS 80 

Clerk  and  Burls'  formula 90 

Ballantyne's  constant 91 

IMPORTANCE   OF   THE   AlR/GAS    RATIO 9! 

Results  of  A.  C.  A.  tests  on  three  cars 92 

R.  A.  C.  Standard  mixture 92 

ADVANTAGES  OF  A  CONSTANT  MIXTURE 93 

M.   I.  T.   experiments  on  explosion  pressures  and  rates  of  flame 

propagation 94 

COINCIDENT  EXISTENCE  OF  FREE  OXYGEN  AND  CARBON  MONOXIDE     .  97 

A  METHOD  OF  ANALYSIS 98 

Method  of  sampling 98 

Leaking  exhaust  pipes,  effect  of 100 

Collecting  the  sample 100 

Transferring  the  sample 100 

Actual  analysis 100 

Determination  of  CO* 102 

Determination  of  O^ 102 

Determination  of  CO 102 

Precautions 102 

CHAPTER   VII 

THE   PHYSICAL   CONDITIONS  OF   CARBURETION 

HEAT 103 

Functions  of .  .  103 

Specific 103 

Latent 103 

Effect  of  evaporation  on  temperature 105 

Effect  of  mixture  proportions  on  temperature 105 

Necessity  for  artificial  heat 106 

Loss  of  volumetric  efficiency  by  heat 106 

Conditions  necessary  for  starting 108 

Effect  of  temperature  on  fuel  flow 109 

PRESSURE no 

Variation  of  compression  pressures no 

Effect  of  reduced  pressures  on  auxiliary  valve 1 1 1 

Effect  of  altitude  on  compression  pressures 1 1 1 

Effect  of  altitude  on  vaporization in 

Air  standard  of  efficiency 113 

Effect  of  altitude  on  power 113 

CHAPTER   VIII 

THE  CARBURETER  OF  THE  FUTURE 

Balanced  forces 114 

Changing  fuel 114 


TABLE    OF    CONTENTS  ix 

PAGE 

Constancy  of  mixture 114 

Atomization 114 

Velocities 115 

Size  and  shape  of  passages 115 

Application  of  heat 115 

Adjustments 116 

Fuel  level 116 

Moving  parts 117 

Accessibility        .            117 

Priming 117 

Fire  protection 117 

Practical  manufacture 118 

Summary 118 


APPENDIX 

USEFUL  TABLES  AND   CONVENIENT  FORMULA 

PAGE 

1.  Absolute  pressure  by  vacuum  gauge         II9 

2.  Mercury  columns IIg 

3.  Compression  efficiency  at  altitudes 119 

4.  Head  in  feet  to  pressure  in  pounds  per  square  inch 120 

5.  Drop  in  pressure  by  velocity 120 

6.  Velocity  of  flow 121 

7.  Manometer  pressures 121 

8.  Displacement  formulae 121 

9.  Speed  formulae .  122 

to.  Volume  ratios  from  weight  ratios 123 

u.  Acceleration  computations .  123 

12.  Computations  of  velocity 123 

13.  Loss  of  pressure  in  pipes 124 

14.  Effect  of  bends 124 

15.  Water,  weight  and  pressure  of 124 

1 6.  Volumetric  efficiency 124 

17.  Brake  horse-power 125 

1 8.  Capacity  of  Prony  brakes 125 

19.  Temperature  correction  for  specific  gravity  of  gasoline     ....  125 

20.  Weight  of  gases 125 

21.  Baume  hydrometer  and  corresponding  specific  gravities   ....  126 

22.  British  thermal  unit 126 

23.  Volume,  pressure,  and  density  of  air 127 


HANDBOOK 
OF   CARBURETION 

CHAPTER  I 

THEORY  OF  CARBURETION 

THE  law  of  the  flow  of  fluids,  including  gases  within  certain 
limits  of  pressure  differences,  is  expressed 


v  =  ^2gh  (i) 

where 

•o  =  velocity  in  feet  per  second. 

g  =  acceleration  of  gravity  (32.2  feet  per  second). 

h  =  head,  or  height  in  feet  of  the  fluid,  required  to  produce 

the  pressure  necessary  to  cause  the  flow. 
The  velocity  of  the  air  (Va)  in  a  carbureter  will  be  expressed 


Va  =  V^A  (2) 

whence 

Fa2  =  2gh  (3) 

and 

*  =  ™  (4) 


In  this  case,  h  is  the  height  in  feet  of  a  column  of  air,  the 
weight  of  which  will  exert  the  pressure  necessary  to  cause  a  flow 
of  air  at  the  velocity  Va,  or  conversely,  the  loss  of  head  caused 
by  the  air  flowing  at  the  velocity  Va. 

The  value  of  h,  or  as  applied  to  carburetion  h',  must  not  be 
understood  to  be  literally  the  vertical  measurement  between 
the  surface  of  the  fuel  in  the  float  reservoir  and  the  mouth  of  the 
fuel  nozzle.  To  this  must  be  added  the  "friction  head"  imposed 
on  the  fuel  by  its  passage  through  the  nozzle.  This  is  subject 

1 


2  HANDBOOK   OF   CARBURETION 

to  constant  variation  and  depends  in  value  upon  the  velocity, 
density,  and  viscosity  of  the  fuel.  The  exact  value  of  hr  is 
probably  indeterminable,  and  so,  for  use  in  the  following  illus- 
trative formulas,  it  will  be  assigned  the  numerical  value  of  the 
vertical  distance,  without  attempt  at  correction. 

The  head  of  fuel  caused  by  air  passing  at  a  velocity    Va 
will  be 


where 

Wa  =  weight  i  cubic  foot  of  air  (.076  pounds  at  62-°  F.). 

Wf  =  weight  i  cubic  foot  of  fuel  (weight  i  cubic  foot  of 

water  j  62.355  [  pounds  X  sp.  gr.  of  fuel). 
Applying  equation  (2)  to  the  fuel  velocity,  Vf,  we  have 


,  Wa 

ishwj 

But  as,  before  actual  discharge  commences,  the  fuel  must  rise 
from  the  level  in  the  float  chamber  to  the  mouth  of  the  fuel 
nozzle,  a  distance  of  h'  feet,  subject  to  the  retardation  of  gravity, 
we  must  deduct  the  value  of  2gh',  and  hence 


:  wf 

Substituting  the  value  of  2gh  as  determined  by  equation  (3), 
the  velocity  of  the  fuel  is  expressed  in  terms  of  air  velocity  as 
follows: 

I",, 

•  Va*  —  2gh'  (5) 

Wimperis  ("  The  Internal  Combustion  Engine,"  page  268) 
arrives  at  the  same  relation  between  air  and  fuel  velocities  by 
methods  of  the  calculus. 

EFFECT  OF  TEMPERATURE 

The  density  of  both  fuel  and  air  is,  of  course,  modified  by 
temperature.  The  density  of  the  air  varies  inversely  as  the 


THEORY   OF   CARBURETION  3 

absolute  temperature,  while  the  density  of  gasoline  is  shown  by 
Clerk  and  Burls  ("The  Gas,  Petrol,  and  Oil  Engine,"  Vol.  II, 
page  623)  to  be  modified  by  temperature  as  follows: 

Sp.  gr.  =  0.72  \ i  —  .0007  (/  —  60)  } 
whence 

Wf  =  W  X  s  \i  -  .0007  (/  -  60) {  (6) 

where 

Wf  =  weight  of  i  cu.  ft.  of  gasoline. 
W  =  weight  of  i  cu.  ft.  of  water, 
j  =  specific  gravity  of  gasoline  at  60°  F. 
/  =  temperature  of  the  gasoline  in  F.° 
t'  =  temperature  of  the  air  in  F.° 

Substituting  these  values  in  equation  (5)  we  have: 

I  (460  +  62)  0.076 

Vf  = 


The  range  of  values  for  t  and  t'  to  be  used  in  equation  (7)  is  so 
small  that  it  will  be  readily  seen  that  the  effect  of  temperature 
is  negligible. 

WORKING  FORMULA 

Omitting  the  temperature  correction,  a  simple  working  equa- 
tion for  gasoline  of  a  specific  gravity  of  0.72  may  be  expressed 


Vf  =  V  (.00169  Va2)  —  2gh' 
For  fuel  of  any  other  gravity,  equation  (5)  becomes 


which  reduces  to 

I      /   r^f\T  s*sy  \ 

'   2gtf  (9) 


HANDBOOK   OF   CARBURETION 


APPLICATION  OF  THE  LAW  TO  VARIOUS  TYPES 

In  order  to  obtain  a  clear  understanding  of  the  application 
of  the  law,  let  us  consider  the  action  of  various  types  of  carbureting 
devices  in  view  of  the  relation  of  air  and  fuel  velocities  as  ex- 
pressed in  equation  (8). 

Hypothesis 

Assume  (A)  that  a  unit  quantity  of  air  is  passing  each  devi.ce 
with  a  given  velocity  and  then  (B)  that  a  greater  quantity  of 
air  is  demanded.  For  the  sake  of  uniformity  let  us  assume 
that  each  device  maintains  a  constant  level  of  fuel  0.5  inch 
(0.0416  feet)  below  the  mouth  of  the  fuel  nozzle  and  that  the 
fuel  employed  is  gasoline  of  a  specific  gravity  of  0.72. 

TYPE  I 
THE   SIMPLE   CARBURETER 

(A)  In  this  device,  the  velocity  of  the  fuel  discharge  for  an 
air  velocity  of  say  90  feet  per  second  will  be,  by  equation  (8) 


Vf  =  V  (.ooi69X902)  -  (644X  .0416X0.72)  =  3.43  ft.  per  sec. 

(B)  As  the  area  of  air  admission  is  constant,  four  times  the 
air  will  pass  at  four  times  the  velocity.  By  equation  (8)  this 
will  induce  a  fuel  flow  of 


Vf  =  V  (.00169  *  3^°2)  ~  J-9  =  I4-73  ft.  per  sec. 

Tendency  Toward  Enrichment 

Hence,  while  the  quantity  of  air  has  been  increased  four 
times,  the  quantity  of  fuel  has  increased  4.4  tunes  and  the 
resulting  mixture  is  10.4  per  cent  richer  than  formerly. 

TYPE  II 

THE  MIXING  VALVE 

In  this  device,  head,  pressure  on  the  valve,  amount  of  valve 
opening,  admission  area  exposed  by  said  opening,  and  the  quan- 


THEORY   OF   CARBURETION  5 

tity  of  air  admitted  are  in  direct  proportion  to  one  another,  if 
friction  is  disregarded.  It  follows  therefore,  that,  as  the  head 
varies  with  the  square  of  the  velocity  (equation  4),  the  quantity 
of  air  bears  the  same  relationship.  Conversely  we  may  state 
that  the  velocity  varies  as  the  square  root  of  the  quantity  of  air 
admitted. 

(A)  The  fuel  flow,  for  an  air  velocity  of  90  feet  per  second, 
will  be  3.43  feet  per  second  as  in  (I-A). 

Tendency  Toward  Impoverishment 

(B)  By  the  proportion  stated  above,  four  times  the  initial 
quantity  of  air  will  pass  the  apparatus  at  twice  the  initial  velocity. 
Hence  the  fuel  flow  induced  by  the  increased  quantity  will  be, 
by  equation  (8) 


Vf  =  V  (.00169  X  i8o2)  —  1.9    =  7.27  ft.  per  sec. 

showing  that  while  the  air  quantity  has  increased  four  times, 
the  fuel  quantity  has  increased  only  2.17  times,  or  but  54  per 
cent  of  the  fuel  is  present  that  is  necessary  for  a  constant  mixture. 
It  is  thus  readily  seen  why  the  mixing  valve  cannot  be  used  for 
carburetion  where  any  material  degree  of  flexibility  is  desired. 

TYPE  HI 
THE   COMPENSATING   CARBURETER 

Attempts  to  correct  the  tendency  to  over-richness  exhibited 
by  the  simple  carbureter  led  to  the  early  adoption  of  the  auxili- 
ary air-valve.  The  popular  conception  of  the  auxiliary  air- 
inlet  is  that  the  air  thus  admitted  serves  to  dilute  the  necessarily 
over-rich  mixture  formed  at  the  mouth  of  the  fuel  nozzle.  As 
all  the  air  entering  the  carbureter,  through  either  the  primary 
or  auxiliary  inlet,  finally  reaches  the  cylinders  as  part  of  the 
explosive  mixture,  the  foregoing  statement  is  obviously  true, 
but  the  most  important  function  of  the  auxiliary  inlet  is  likely 
to  be  lost  sight  of  in  such  an  explanation  of  its  purpose. 


6  HANDBOOK   OF  CARBURETION 

True  Function  of  the  Auxiliary 

The  area  of  the  auxiliary  opening  modifies  the  velocity  of 
all  the  incoming  air  and  hence  exercises  a  direct  influence  upon 
the  amount  of  fuel  inspirated.  This  function  will  be  better 
understood  if  the  primary  and  auxiliary  inlets  are  considered 
as  a  divided  unit.  Any  enlargement  of  the  auxiliary  area 
increases  the  total  area  of  admission  and  hence  modifies  both 
quantity  and  velocity. 

In  a  carbureter  of  this  type  let 

Q  =  the  quantity  of  air. 
V  =  velocity  of  the  air. 

a  =  auxiliary  area. 

c  =  the  primary  area. 
A  =  total  admission  area  =  a  -\-  c. 

g  =  acceleration  of  gravity. 

Disregarding  friction,  the  quantity  of  fluid  discharged  by  an 
orifice  is  expressed 

Q  =  VA  (10) 

Hence  the  quantity  of  air  passing  the  carbureter  will  be 
Q  =  V  (a  +  c) 

which,  by  substituting  the  value  of  V  from  equation  (i),  may  be 
written 

Q=  (a  +  c)  ^2gh  (n) 


In  this  equation  h  is  the  height  of  a  column  of  air  necessary  to 
cause  a  unit  deflection  of  the  spring  governing  the  auxiliary 
valve;  therefore  the  velocity  of  a  given  quantity  of  air  is  directly 
dependent  upon  spring  tension  and  deflection,  as  well  as  upon 
the  relative  areas  of  both  primary  and  auxiliary  openings.  As 
these  variables  are  fixed  by  construction,  determination  of  the 
quantity  and  velocity  may  be  effected  by  simple  substitution 
of  the  known  values  in  equation  (n). 


THEORY   OF   CARBURETION  7 

For  instance,  assume  that  in  a  carbureter  of  this  type, 
provided  with  a  primary  inlet  fg  inch  in  diameter  (0.3  square 
inch  area),  a  vacuum  of  i  inch  of  water  causes  an  auxiliary  area 
of  0.05  square  inch  to  be  opened. 

(A)  A  head  of  i  inch  of  water  is  equivalent  to  a  he  ad  of 
68.284  feet  of  air  at  normal  pressure  and  temperature. 

By  equation  (i) 


V  =  V644  X  68.28  =  66.31  feet  (or  796  inches)  per  second. 

A  =  0.3  +  .05  =  0.35  square  inch. 

Q  =  796  X  0.35  =  278.6  cubic  inches  per  second. 

By  equation  (8) 


Vf  =  V(.ooi69  X  66.3 12)  —  1.9  =  2.35  feet  per  second. 

(B)  Assume  now,  that  on  open  throttle,  the  vacuum  within 
the  carbureter  is  20  inches  of  water.  The  head  of  air  would  be 
20  X  68.28  =  1,365.6  feet. 


V  —  V644  X  1,365.6  =  296.5    feet    (or  3,559  inches)  per 

second. 

A  —  0.3  +  (0.5  X  20)  =  1.3  square  inches. 
Q  =  3>559  X  1.3  =  4,616.7  cubic  inches  per  second. 
Vf  —  V(.ooi69  X  296. 52)  —  1.9  =  12. i  feet  per  second. 
Tendency  Toward  Impoverishment 

Therefore,  the  air  flow  has  increased  - —  =  16.1;  times  while 

279 

the  fuel  flow  has  increased  only =  5.15  times;  or  but  31  per 

cent  of  the  former  proportion  of  fuel  is  present.  In  other  words, 
had  the  original  mixture  in  (A)  been  in  the  air/gas  ratio  of  say 
10/1,  the  high-speed  mixture  of  (B)  would  be  in  the  ratio  of 
32/1,  which  is  far  beyond  the  limits  of  combustibility. 

Failure  of  Corrective  Devices 

As  may  be  readily  determined,  no  adjustment  of  spring 
tension  can  do  more  than  very  slightly  modify  this  tendency 

\ 


8  HANDBOOK   OF   CARBURETION 

toward  impoverishment  of  the  mixture,  while  the  addition  of 
various  forms  of  subsidiary  springs,  becoming  operative  only 
at  some  point  of  the  valve-opening,  can  do  no  more  than  correct 
the  error  at  one  given  point  and  then  start,  as  it  were,  merely  a 
new  scale  of  errors. 

The  inherent  error  of  the  auxiliary  valve  is  by  no  means  of 
theoretical  interest  only.  It  still  remains  a  factor  of  so  intensely 
practical  effect,  despite  the  remarkable  ingenuity  that  has  been 
displayed  in  various  attempts  to  correct  it,  that  its  elimination 
would  effect  an  annual  saving  of  thousands  of  dollars  to  both 
manufacturer  and  user  of  motor-cars  through  the  increased 
efficiency  of  the  liquid  fuel  engine. 

TYPE  IV 
THE   MULTIPLE  JET  CARBURETER 

Attempts  to  correct  the  error  in  mixture  composition  intro- 
duced by  the  increasing  air  flow  have  been  confined  largely  to 
two  principal  channels.  Abroad,  the  tendency  is  toward  the  use 
of  multiple  fuel  jets,  while  in  this  country  more  attention  has 
perhaps  been  given  to  the  direct  mechanical  regulation  of  the 
area  of  the  orifice  in  the  fuel  nozzle. 

Each  Succeeding  Jet  Subject  to  Error  of  Type  I 

It  will  be  apparent  from  the  foregoing  treatment  of  the 
subject  that,  in  multiple  jet  practice,  the  flow  from  each  suc- 
ceeding jet  is,  in  turn,  amenable  to  the  law  of  fluid  flow  as 
expressed  in  equation  (8).  Hence,  each  succeeding  jet,  like 
the  subsidiary  spring  on  the  auxiliary  valve  of  Type  III  merely 
corrects  the  error  at  the  point  where  its  own  discharge  com- 
mences and  then  the  flow  suffers  a  cumulative  error  until  corrected 
by  the  introduction  of  the  flow  from  still  another  jet. 

It  is  evident  that  the  use  of  a  sufficient  number  of  jets  might 
be  made  to  reduce  the  error  to  very  small  proportions,  and  in 
fact  good  results  have  been  obtained  from  such  construction. 
Mechanical  complications  and  the  nicety  of  constructional 
detail  have  proved  serious  disadvantages,  however. 


THEORY   OF   CARBURETION 


TYPE    V 
THE   VARIABLE   FUEL   ORIFICE 

Inspection  of  equation  (8)  and  the  substitution  of  values 
therein  in  the  examples  cited  disclose  that  the  fuel  velocity  is 
in  constantly  decreasing  proportion  to  the  air  velocity.  In 
Type  III,  the  quantity  of  fuel  discharge  has  been  treated  of  in 
terms  of  fuel  velocity.  It  is  evident,  however,  from  equation 
(10)  that  the  actual  fuel  discharge  is  the  product  of  its  velocity 
and  the  area  of  the  fuel  orifice.  Hence,  it  will  be  recognized 
that  variation  of  the  area  of  fuel  orifice  may  be  made  to  com- 
pensate for  the  increasing  ratio  between  the  fuel  and  air  velocities. 
In  III-B,  for  instance,  while  the  quantity  of  air  was  increased 
16.5  times,  the  fuel  velocity  increased  only  5.15  times;  therefore, 
to  maintain  constancy  of  mixture,  the  area  of  the  fuel  orifice 

16.5 
should  have  been  increased  --  =  3.2  times. 


Delicacy  of  Construction  and  Adjustment 

The  withdrawal  of  a  straight  tapered  pin  from  the  fuel  nozzle 
increases  the  area  of  discharge  in  direct  proportion  to  the  lift 
of  the  pin;  consequently,  delicate  mechanical  complications  are 
resorted  to  in  effecting  the  desired  decrease  in  the  proportional 
area  opened.  Properly  designed  and  properly  adjusted,  there 
is  no  reason  why  this  method  should  not  give  results  approaching 
accuracy,  but  when  we  consider  the  almost  microscopic  nicety 
of  adjustment  necessary  to  effect  accurate  sub-division  of  the 
minute  fuel  stream,  we  realize  the  practical  difficulty  of  both 
making  and  maintaining  such  adjustments.  When  we  remember, 
too,  that  the  volume  of  liquid  gasoline  is  less  than  1/8000  of  the 
volume  of  the  air  with  which  it  is  mixed,  it  is  apparent  that 
regulation  of  the  8,000  parts  would  be  much  more  practical  than 
any  attempt  to  subdivide  the  one  part. 


10  HANDBOOK    OF    CARBURETION 

TYPE    VI 
THE   CONSTANT   VACUUM  PRINCIPLE 

If  air  is  admitted  to  a  chamber  through  an  opening  which  is 
governed  by  a  weighted  valve,  a  sub-atmospheric  pressure  or 
partial  vacuum  will  be  maintained  in  the  chamber,  equivalent 
to  the  weight  of  the  valve  per  square  inch  of  exposed  area. 

As  the  demand  for  air  becomes  greater  the  valve  will  be  lifted 
higher,  admitting  just  enough  air  to  maintain  a  vacuum  in 
consonance  with  the  weight  of  the  valve,  which  is,  of  course, 
constant  at  all  times. 

Comparison  With  Type  V 

The  vacuum  being  constant,  it  follows  that  the  velocity  of 
the  entering  air  is  constant  and  hence  it  is  necessary  to  provide 
some  means  for  increasing  the  area  of  the  fuel  flow.  As  in 
Type  V,  this  may  be  accomplished  by  withdrawing  a  tapered 
needle  from  the  fuel  nozzle.  In  this  respect  this  type  has  the 
advantage  over  Type  V,  because  the  increase  in  fuel  flow  is  a 
straight  line  curve  and  hence  the  proportional  withdrawal  of  a 
straight  tapered  needle  maintains  constant  proportions  of  flow. 
The  needle  may  be  therefore  directly  attached  to  the  air- valve, 
and  move  with  it. 

If  the  taper  is  properly  calculated  to  allow  for  decreasing 
friction  as  the  opening  becomes  greater,  this  device  should 
maintain  constancy  of  any  given  mixture  proportions. 

Acceleration 

During  the  brief  instant  when  the  air-valve  is  actually  being 
lifted,  as  on  opening  the  throttle,  the  vacuum  is  temporarily 
increased,  because  more  energy  is  necessary  to  move  the  valve 
than  to  sustain  it  in  a  given  position.  The  result  is  a  slight 
additional  impetus  given  to  the  fuel  flow.  A  valve  can  be 
designed  of  such  weight  that  this  tendency  to  increase  the 
richness  of  the  mixture  is  nearly,  or  quite,  counter-balanced 
by  the  inertia  of  the  fuel.  Such  a  design  gives  prompt  and 
very  satisfactory  acceleration. 


THEORY   OF   CARBURETION  11 

Constancy  cannot  be  maintained,  however,  when  the  mixture 
proportions  are  to  be  varied  by  "adjusting  the  needle  relative 
to  the  nozzle,  because,  as  the  areas  of  circles  vary  as  the  square 
of  their  diameters,  the  annulus  between  the  needle  and  the 
nozzle  at  minimum  opening  is  directly  proportional  to  the 
corresponding  annulus  at  full  opening,  only  when  the  needle  is  in 
the  position  for  which  the  areas  were  determined. 

Advantages 

This  type  presents  the  distinct  advantages  of: 

First. — Relatively  high  velocities  at  low  speeds,  insuring 
comparative  ease  of  starting,  and  making  slow  running  possible 
either  on  full  or  part  throttle. 

Second. — No  increase  of  velocity  at  extreme  high  speeds, 
hence  no  reduction  of  volumetric  efficiency. 

Third. — Comparative  freedom  from  the  effects  of  barometric 
changes. 

TYPE  VII 

THE   COMPENSATING  NOZZLE 

As  has  been  shown  in  Type  I,  the  tendency  of  a  simple  fuel 
jet  in  a  moving  air  column  is  toward. enrichment.  Attempts 
have  been  made  to  correct  this  tendency  by  using  a  second 
nozzle,  which  receives  a  limited  flow  of  fuel  from  an  orifice  of 
such  area  that,  as  the  air  velocity  increases,  insufficient  fuel  is 
delivered  and  the  resulting  mixture  becomes  leaner.  By  com- 
bining the  two  nozzles,  the  first  with  its  tendency  to  enrichment 
and  the  second  with  its  tendency  to  impoverishment,  it  is  claimed 
that  a  balance  is  established  which  produces  a  constant  mixture. 

Difference  in  Governing  Laws 

The  discharge  from  the  enriching  nozzle  follows  the  law  of 
fluid  flow,  while  the  action  of  the  compensating  nozzle  is  de- 
pendent solely  on  the  friction  on  the  fuel  in  passing  into  the 
nozzle.  It  is  doubtful,  therefore,  if  the  reaction  between  the 
two  nozzles  is  more  than  an  approximation  to  true  compensation, 


12  HANDBOOK   OF   CARBURETION 

Instruments  of  this  type  are  widely  used  in  European  practice, 
and  to  a  considerable  extent  in  this  country.  Their  freedom 
from  moving  parts  is  attractive,  and  their  performance  is  as 
good  as  that  of  many  other  types. 

Close  Adjustment  Necessary 

They  are  subject,  however,  to  several  disadvantages: 
First. — Because   the   compensation  is  effected  by  friction, 
any  approach   to   accuracy  is  confined  within  comparatively 
narrow  limits  of  air  quantities,  and  is  even  then  obtained  only 
with  minute  accuracy  of  workmanship  and  final  adjustment. 

Additional  Starting  Device  Required 

Second. — The  air  for  all  speeds  is  admitted  through  a  single 
opening  of  fixed  area.  This  area  must  be  sufficiently  large  to 
prevent  undue  friction,  or  wire-drawing  at  high  speeds.  In 
consequence,  it  must  be  too  large  to  insure  proper  atomizing 
velocities  at  starting  speeds.  The  latter  is  commonly  provided 
for  by  a  third  jet  inserted  near  the  edge  of  the  butterfly  throttle. 
In  effect,  this  is  a  separate  carbureter,  operative  only  when  the 
throttle  is  nearly  closed. 

Action  on  Full  Throttle 

Third. — Because  of  the  fixed  area  of  the  air-inlet,  it  is  to  be 
expected  that  flexibility  on  open  throttle  will  be  sacrificed. 
Either  maximum  speed  will  be  curtailed  by  wire-drawing,  or  the 
engine  will  not  run  slowly  under  heavy  load  and  full  throttle 
opening. 

Acceleration 

Fourth. — As  the  density  of  fuel  is  greater  than  that  of  air 
so  is  its  inertia  increased.  As  a  result,  upon  suddenly  opening 
the  throttle  for  acceleration,  the  mixture  is  momentarily  im- 
poverished as  the  air  flow  exceeds  that  of  the  fuel.  The  result 
is  that  this  type  does  not  give  that  instant  response  to  the  throttle 
that  is  desirable. 


THEORY   OF   CARBURETION  13 

TYPE    VIII 
COMPENSATION   BY  VELOCITIES 

It  has  been  shown  in  Type  V  that  compensation  can  be 
effected  by  the  variation  of  the  area  of  the  fuel  nozzle.  It  is 
equally  true  that  automatic  variation  of  the  total  air  admission 
area  will  accomplish  the  same  result  with  much  greater  accuracy 
and  without  adjustments  or  mechanical  complications  of  any 
kind.  For  this  purpose  it  is  necessary  to  determine  the  velocity 
of  the  air  corresponding  to  any  given  fuel  velocity. 

If,  by  equation  (8) 


Vf  =  V  (. 


or,  more  conveniently, 


Va  =  V59i.7i  (F/2  +  2g/0  (12) 

The  practical  application  of  these  formulae  is,  perhaps,  best 
made  clear  by  a  concrete  example.  Let  us  consider  a  carbureter 
with  a  primary  inlet  5/l6  inch  in  diameter  (area,  0.077  square 
inch).  Let  us  assume  the  auxiliary  valve  to  be  governed  by  a 
spring  that  will  deflect  o.oi  inch  for  a  vacuum  in  the  carbureter 
of  i  inch  of  water. 

(A)  Assume  that  230  cubic  inches  of  air  per  second  are  pass- 
ing through  this  carbureter  at  a  velocity  of  90  feet  per  second. 
By  equation  (8)  the  fuel  velocity  will  be 

Vf  =  V  (.00169  x  9°2)  —  1-9  ==  3-43  feet  per  second, 
the  vacuum  will  be 

— ..  ,0 — -  =1.84  inches  of  water. 

64.4  X  68.28 

The  deflection  of  the  valve  will  be 

o.oi  X  1.84  =  0.0184  inches. 


14  HANDBOOK   OF   CARBURETION 

The  total  admission  area  will  be 

230 

=  0.213  square  inch. 

90  X  12 

The  auxiliary  area  will  be 

0.213  ~~  0.077  =  0.136  square  inch. 

(B)  Assume  now  that  ten  times  the  original  quantity  of  air 
is  demanded. 
The  quantity  of  air  would  be 

230  X  10  =  2,300  cubic  inches  per  second. 

This  air  must  pass  the  fuel  jet  with  a  velocity  sufficient  to  induce 
a  flow  ten  times  the  initial  quantity  of  the  fuel. 
As,  by  equation  (12) 


the  air  velocity  that  will  increase  the  fuel  flow  ten  times  may  be 
expressed 

Vau  =  V59i.7i  j 


Substituting  the  values  of  the  present  example 


Va10  =  V59I-7I  {io(342)  +  1.9}=  263.67  feet  per  second. 

2642 

The  vacuum  will  be  - —    — — -  =  ic.Ss  inches  of  water. 
64.4  X  62.28 

The  deflection  of  the  valve 

15.85  X  o.oi  =  0.158  inch. 
The  total  admission  area 

^^=0.73  square  inch. 

The  auxiliary  area 

0.73  — 0.077  =  °-^53  square  inch. 


THEORY   OF   CARBURETION  15 

Working  Equations 

As  a  practical  convenience  these  equations  may  be  simplified 
and  expressed  in  terms  of  fuel  velocity  as  follows: 


Velocity  of  the  air  =  24.32  VF/"2  +  2gh!  (13) 

Total  admission  area  =  —  (14) 

292  VFf2  +  2°ti 

F/"2  +  2gh' 

Vacuum  in  inches  of  water  =  —  (15) 

7-44 

Total  spring  deflection  = 2LJ^  ^ 

7-44 

where  d  =  the  spring  deflection  for  a  vacuum  of  i  inch  of  water. 
By  the  use  of  these  formulae  the  auxiliary  air  admission  area 
may  be  determined  for  any  number  of  points  in  the  travel  of 
the  valve  and  the  walls  surrounding  the  valve  may  be  made  to 
conform  to  the  curve  so  plotted,  thus  assuring  the  permanent 
maintenance  of  any  desired  air/gas  ratio  without  adjustments 
of  any  kind. 

RELATION  OF  VELOCITY  TO  VACUUM 
Friction 

In  all  the  foregoing  calculations  the  influence  of  friction  and 
other  factors  modifying  the  flow  of  liquids  in  a  carbureter  have 
been  omitted  for  the  purpose  of  permitting  simplified  statements 
of  fundamental  principles.  These  modifications  are,  however, 
of  prime  importance,  none  the  less  so  because  their  variant 
values  are  undetermined.  They  affect  the  flow  of  both  fuel 
and  air  to  such  an  extent  that,  without  giving  them  due  con- 
sideration, the  application  of  any  formulas  expressing  the  re- 
lationship of  actual  flow  of  fuel  and  air  would  be  impossible. 

Instrumental  Elimination  of  Unknown  Quantities. — Barometric 

Effects. — Inherent  Accuracy 

Thus  the  formulae  herein  expressed  have,  so  far,  tentatively 
assumed  that  the  drop  in  pressure  or  "vacuum"  at  the  mouth 


16  HANDBOOK   OF    CARBURETION 

of  the  fuel  nozzle  was  the  same  as  that  within  the  mixing  chamber. 
Repeated  experiments  have  demonstrated  the  fallacy  of  such  an 
assumption,  to  which  indeed  must  be  attributed  the  failure  of 
many  otherwise  meritorious  devices.  Solution  of  the  intricate 
problems  existing  between  the  mouth  of  the  fuel  nozzle  and  the 
mixing  chamber,  involving  marked  physical  changes  in  both 
the  liquid  fuel  and  the  air,  would  be  interesting  theoretically, 
but,  from  a  practical  standpoint,  we  are  fortunately  able  to 
eliminate  the  effect  of  these  modifying  influences  instrumentally. 
This  can  be  accomplished  by  two  structural  modifications. 
First,  the  control  of  the  auxiliary  area  directly  by  the  vacuum 
at  the  mouth  of  the  fuel  nozzle,  which  construction  also  presents 
the  further  practical  advantage  of  rendering  the  action  of  the 
instrument  practically  insusceptible  to  barometric  changes. 
Second,  by  a  slight  modification  of  the  curve  of  auxiliary- 
admission  areas,  so  that  the  air  velocities  are  increased  to  a 
sufficient  amount,  determined  experimentally,  to  compensate  for 
the  frictional  resistance  offered  by  the  nozzle  to  the  flow  of  the 
fuel.  Instruments  constructed  in  accordance  with  the  foregoing 
principles  have  been  found  to  maintain  a  constancy  of  mixture 
in  strict  accord  with  the  theory,  and  it  has  been  determined  that 
the  slightest  departure  from  the  theoretical  curve  of  admission 
areas  produces  negative  results  in  constancy  of  composition. 

VARIABLE  MIXTURES 
Modification  of  the  Auxiliary  Curoe 

If,  however,  it  were  desirable  to  vary  the  mixture  composi- 
tion for  different  operating  conditions,  the  proposed  method 
lends  itself  readily  to  that  end.  Thus,  the  auxiliary  areas  may 
be  diminished  at  and  near  the  starting  end  of  the  curve,  resulting 
in  the  richer  mixture  so  often  claimed  to  be  necessary  for  easy 
starting.  At  ordinary  road  speeds  the  areas  may  be  so  calcu- 
lated that  a  mixture  of  high  fuel  economy  will  result,  while  at 
extreme  open-throttle  for  high  speed,  contraction  of  the  ad- 
mission curve  will  increase  the  richness  of  the  mixture  for  the 
development  of  maximum  power.  In  other  words,  the  designer 


THEORY   OF   CARBURETION  17 

has  but  to  determine  the  range  of  mixture  composition  which  he 
considers  most  satisfactory  and  construct  the  admission  curve  in 
accordance  therewith,  knowing  that  whatever  action  has  been 
selected  will  be  repeated  with  invariable  exactitude. 

CONSTANT  MIXTURE 
Advantages 

The  results  obtained  from  many  different  engines  by  the  use 
of  gasoline  mixtures  of  really  constant  composition  have  been  so 
pronounced  as  to  be  in  the  nature  of  a  revelation,  particularly 
as  regards  certain  details  not  ordinarily  considered  as  primary 
functions  of  carburetion.  There  is  noticeable  a  marked  quietness 
of  operation  noj:  easily  explained,  unless,  possibly,  the  uniform 
rate  of  flame  propagation  establishes  a  rhythmical  vibratory 
effect.  The  objectionable  features  of  fluctuating  mixtures  are, 
naturally,  minimized.  After  a  full  season's  running  the  cylinders 
of  several  cars  were  found  free  from  carbon,  while  the  spark- 
plug points  were  clean  and  the  porcelains  discolored  by  heat  only. 
Exhaust  gas  analysis  shows  practically  no  loss  through  incom- 
plete combustion.  The  average  of  44  samples  taken  from 
several  different  cars  under  all  sorts  of  road  conditions  gave  0.43 
per  cent  CO,  while  29  samples  yielded  no  CO. 

Velocity  the  Only  Constant 

Governing  the  fuel  flow  by  the  velocity  of  the  entering  air 
seems  to  be  an  ideal  method  for  constancy.  At  a  given  number 
of  revolutions  per  minute  a  given  engine  invariably  takes  the 
charge  into  its  cylinders  at  a  definite  velocity.  Velocity,  however, 
is  the  only  fixed  quantity.  Chemical  composition,  pressure, 
temperature  and,  consequently,  density  of  the  charge,  may  vary 
widely,  but  whatever  the  nature  of  the  charge — whether  it  be 
the  rarefied  atmosphere  of  the  mountain -top  or  the  dense  fog 
of  the  seaboard — at  a  given  engine  speed  the  cylinder  is  filled 
(according  to  its  volumetric  efficiency  under  the  conditions)  jn 
the  same  interval  of  time.  Velocity  is  a  constant,  and  upon  it 


18  HANDBOOK  OF   CARBURETION 

and  it  alone  may  be  safely  based  the  computations  necessary 
for  accurate  metering  of  the  fuel. 

Loss  OF  VOLUMETRIC  EFFICIENCY 

The  practical  operation  of  this  type  entails  that  increased 
air  quantities  be  admitted  at  velocities  sufficiently  increased,  so 
that  the  proper  amount  of  fuel  be  inspirated. 

Low  Speed  Velocities 

To  insure  ease  of  starting,  an  initial  velocity  of  the  entering 
air  of  90  feet  per  second  is  desirable,  although  with  a  properly 
designed  manifold  this  may  be  safely  reduced  to  60  feet  per 
second.  This  induces  a  fuel  velocity  as  follows: 

By  equation  (8) 


Vf  =  V  (.00169  X  6o2)  —  1.9  =  2.0455  feet  per  second 

High  Speed  Velocities 

Assuming  the  engine  at  maximum  speed  requires  15  times 
the  initial  quantity  of  air,  and  consequently  15  times  the  initial 
flow  of  fuel,  necessitating  a  velocity  of 

15  X  2.04552  =  62.76  feet  per  second. 
By  equation  (13) 


Velocity   of   the   air  =  24.32  >  67.76  +  c  +  1.9  where  c  is   a 
coefficient  of  friction. 

If,  for  illustration,  we  assign  to  c  a  value  of  .04  we  have 


Va  =  24.32  V  65.2  +  1.9  =  199.32  feet  per  second. 

By  Chart  I,  this  velocity  is  seen  to  represent  a  volumetric  loss 
of  about  2  per  cent.  With  an  initial  velocity  of  about  90  feet  per 
second  this  loss  is  about  6.2  per  cent. 

Volumetric  Loss 

To  these  losses  must  be  added  the  loss  by  friction  in  carbureter 
and  manifold,  and  it  is  evident  that  practical  design  must 


THEORY   OF   CARBURETION  19 

recognize  these  volumetric  losses  and  select  the  range  of  velocities 
accordingly. 

SUMMARY  OF  TYPES 

TYPE  I.  Non-Compensating. — Produces  an  enriched  mixture  as 

speed  increases. 
TYPE  II.  Mixing  Valve. — Produces  a  leaner  mixture  as  speed 

increases. 
TYPE  III.  Vacuum  Operated  Auxiliary  Valve — Has  an  inherent 

tendency  to  impoverishment,   which  cannot  be  accurately 

corrected. 
TYPE  IV.  Multiple  Fuel  Jets.— Each  jet  subject  to  the  same 

conditions  as  Type  I. 
TYPE  V.  Variable  Fuel  Orifice. — May  produce  constant  mixture 

but  entails  delicacy  of  adjustment. 
TYPE  VI.  Constant  Vacuum. — May  produce  constant  mixture 

of  one  definite  air/gas  ratio,  but  entails  mechanical  difficulties 

if  constancy  is  to  be  maintained  where  ratio  is  changed. 
TYPE   VII.  Compensating   Nozzle. — Compensation    effected   by 

dissimilar  laws.    Limited  speed  range  on  full  throttle  and 

inherent  difficulties  of  acceleration. 
TYPE  VIII.  Compensation  by  Velocity. — Constancy  of  mixture 

and  freedom  from  barometric  changes  obtainable.     Slight 

decrease  of  volumetric  efficiency  at  high  speeds  unavoidable. 


CHAPTER  II 
THE  INTAKE  MANIFOLD 

THE  functions  of  the  intake  manifold  are  so  closely  allied 
with  those  of  the  carbureter  as  to  be  inseparable  in  any  detailed 
study  of  the  science  of  carburetion. 

Functions  of  the  Carbureter  and  Manifold 

With  the  fuels  of  the  present  day,  the  carbureter  proper  does 
little  else  than  to  proportion  the  amount  of  liquid  fuel  delivered 
to  the  air.  Thus,  it  may  be  stated  that  the  carbureter  is  re- 
sponsible for  the  chemical  composition  of  the  mixture,  while 
the  physical  condition  of  the  charge  is  dependent  upon  subse- 
quent processes  of  gasification  and  diffusion  taking  place  very 
largely  within  the  intake  manifold,  the  valve  chambers,  and 
even  within  the  cylinder  itself. 

The  design  of  the  intake  manifold  and  its  effect  on  the 
physical  characteristics  of  the  charge,  therefore,  become  an 
essential  part  of  the  problem  of  carburetion. 

PROBLEMS  INVOLVED 

Design  of  the  intake  manifold  of  the  liquid  engine  presents 
two  problems:  First,  that  each  cylinder  receive  an  equal  quan- 
tity of  mixture;  second,  that  the  mixture  reaching  each  cylinder 
shall  possess  the  same  chemical  and  physical  characteristics. 
These  factors  are  of  much  greater  importance  in  the  smoothness 
of  operation  and  general  efficiency  of  the  engine  than  is  commonly 
recognized. 

THE  CONDITIONS 
Product  of  Carbureter,  Mist — Not  Gas 

Were  the  product  of  the  carbureter  a  homogeneous  gas,  the 
problem  of  manifold  design  would  be  largely  a  matter  of  con- 

20 


THE   INTAKE   MANIFOLD  21 

venient  dimensions  and  proportionate  branchings.  In  fact, 
when  the  grade  of  commercial  gasoline  was  much  lighter  than 
it  is,  the  manifold  presented  few  problems.  The  prevailing 
grade  of  gasoline  and  its  constant  degeneration,  coupled  with 
the  commercial  necessity  of  using  fuels  of  still  lower  volatility, 
make  the  manifold  an  active  and  important  adjunct  of  the 
carbureter.  The  mixture  leaving  the  throat  of  the  carbureter 
is  by  no  means  a  true  gas,  but  consists  chiefly  of  liquid  particles 
carried  in  mechanical  suspension  in  the  moving  air  current. 
From  the  moment  of  admixture,  these  particles  undergo  con- 
stant evaporation.  With  the  highly  volatile  fuels  formerly 
obtainable,  the  reduced  velocities  through  an  enlarged  area  in 
the  mixing  chamber  of  the  carbureter  afforded  sufficient  time 
to  convert  these  particles  almost,  if  not  wholly,  to  gas.  With 
the  less  volatile  fuels  of  to-day  the  time  factor  of  unaided  evapora- 
tion is  so  great  that  a  considerable  portion  of  the  fuel  traverses 
the  greater  part,  if  not  the  entire  length,  of  the  manifold  as  a 
mist  suspended  in  the  air  current. 

VELOCITIES  IN  THE  MANIFOLD 
Necessities  for  Proper  Velocities 

A  definite  velocity  is  required  to  maintain  this  suspension, 
dependent  upon  the  size  of  the  liquid  particles,  which,  in  turn, 
depends  upon  the  atomizing  force  to  which  the  fuel  has  been 
subjected.  The  moment  the  speed  of  the  moving  air  current  is 
decreased  below  this  critical  velocity,  the  larger  particles  are 
deposited  and  the  mixture  no  longer  contains  the  proportion  of 
fuel  that  was  so  carefully  metered  into  it  by  the  carbureter. 

Any  enlargement  of  the  cross-sectional  area  traversed  by 
the  mixture  decreases  its  velocity,  and  hence,  if,  as  in  starting, 
the  fuel  mist  is  to  be  carried  to  the  cylinders  as  such,  the  diam- 
eter of  the  manifold  would  be  confined  to  narrow  limits. 

A  liquid  fuel  engine  of  average  flexibility  requires  at  least 
from  twelve  to  fifteen  times  its  minimum  amount  of  air  at  maxi- 
mum speed.  If  a  velocity  of,  say,  30  feet  per  second  is  necessary 
to  maintain  the  suspension  of  fuel  atomized  to  a  given  fineness, 


22 


HANDBOOK   OF   CARBURETION 


and  if  the  area  of  the  manifold  is  such  that  this  velocity  is  to 
be  maintained  at  the  lowest  speed,  then  at  maximum  speed  the 
velocity  would  approach  450  feet  per  second.  This  would  entail 


VOLUMETRIC  LOSS 
BY 
VELOCITY 

/ 

/ 

/ 

/ 

7 

/ 

X 

ICO 


200  300 

Ft.  per  Sec. 

CHART  I. 


a  loss  of  volumetric  efficiency  from  velocity  head  alone  of  over 
ii  per  cent,  without  considering  friction  which  would  increase 
this  loss. 

DEPOSITION  OF  LIQUID  FUEL 
Deposition  Not  Condensation 

Furthermore,  whenever  a  moving  fluid  touches  foreign 
surfaces  the  velocity  of  the  surface  stratum  is  markedly  dimin- 
ished by  the  friction  of  the  contact,  called  "skin-friction,"  an 
amount  dependent  upon  the  condition  of  smoothness  of  the 
frictional  surface.  Hence,  that  portion  of  the  air  column  which 
touches  the  walls  of  the  manifold  frequently  falls  below  the 
critical  velocity,  even  though  the  interior  of  the  column  may 
be  maintained  well  above  it.  The  result  is  the  well-known 
wetting  of  all  surfaces,  commonly,  but  erroneously,  attributed 
to  condensation.  Condensation  implies  a  change  of  state  from 


THE   INTAKE   MANIFOLD  23 

a  gas  to  a  liquid.  As  the  fuel  has  never  been  a  gas  during  the 
process  under  consideration,  the  term  condensation  is  not  only 
clearly  a  misnomer,  but  misleading  as  to  actual  conditions  and 
causes. 

From  the  foregoing  it  is  evident  that  it  is  wholly  impractical 
to  depend  upon  high  velocities  within  the  manifold  for  either  the 
quantitative  or  the  qualitative  maintenance  of  the  mixture. 
We  recognize,  then,  that  there  is  and  must  be  a  deposition  of 
liquid  upon  all  interior  surfaces,  depending  in  amount  upon 

(a)  The  degree  of  atomization  within  the  carbureter. 

(b)  The  cross-sectional  area  of  the  manifold. 

(c)  The  form  and  condition  of  smoothness  of  the  manifold 
passages. 

EVAPORATION  OF  DEPOSITED  FUEL 

Before  attempting  design,  let  us  further  consider  what  takes 
place  within  the  manifold.  The  surface  of  the  liquid  film 
wetting  the  walls  is  subjected  to  the  attrition  of  the  moving 
air-column,  with  resulting  evaporation  of  the  liquid.  This 
evaporation  takes  place  only  from  the  surface  of  the  liquid 
and  is  a  relatively  slow  process  with  low  grades  of  fuel.  It  is 
clearly  desirable,  therefore,  to  avoid  pockets  where  any  depth 
of  liquid  can  accumulate,  but,  instead,  to  increase  the  available 
surface  to  the  greatest  possible  extent,  and  hence,  we  hear  of  the 
advisability  of  roughened  interior  walls. 

Surging 

The  alternating  processes  of  deposition  and  evaporation  are 
evidenced  in  the  "surging"  with  which  we  are  familiar  when 
starting  some  engines  on  a  cold  morning.  After  running  a  short 
time,  the  rate  of  evaporation,  assisted  by  the  elevation  of 
temperature  beneath  the  hood,  equalizes  with  the  rate  of 
deposition  and  the  engine  assumes  a  more  even  tenor  of  operation. 

EFFECT  OF  BENDS 

Another  factor  increasing  the  difficulty  of  proper  distribution 
of  a  mist-laden  mixture  is  the  tendency  of  the  liquid  to  seek 


24  HANDBOOK   OF   CARBURETION 

the  outer  periphery  of  all  curves.  However  finely  the  liquid 
may  be  comminuted,  so  long  as  it  remains  a  liquid,  its  specific 
gravity  is  far  greater  than  that  of  the  air,  and,  being  thrown 
violently  against  the  outside  of  the  curve  by  centrifugal  force, 
its  velocity  is  so  lessened  by  the  impact  that  there  is  a  greater 
tendency  to  impoverish  the  mixture  than  would  be  the  case  with 
a  straight  pipe,  or  can  be  accounted  for  by  the  additional  resis- 
tance of  the  curve.  It  follows  that  the  shorter  the  radius  of  the 
curve  the  greater  the  tendency  to  cause  deposition. 

Causes  of  Hard  Starting 

With  the  frequent  enlargement  of  area  and  the  tortuous 
passages  of  many  manifolds,  it  is  probable  that  all  the  un- 
evaporated  liquid  is  deposited  before  reaching  the  cylinders. 
Hence,  an  engine  so  equipped  is  hard  to  start  when  cold.  It  is 
a  common  experience  with  oversized  and  otherwise  poorly 
designed  manifolds  to  observe  an  actual  dripping  from  around 
the  throttle  shaft  and  from  the  primary  inlet  to  the  carbureter, 
after  one  has  become  exhausted  by  ineffectual  cranking  of  a  cold 
engine.  Is  it  any  wonder  that  a  starter  frequently  refuses 
duty? 

METERING  AND  CARBURETION 
Carburetion  Within  the  Manifold 

Under  these  conditions,  as  has  been  noted,  the  carbureter 
functions  chiefly  as  a  metering  device,  while  true  carburetion 
of  the  air  by  fuel  vapor  really  takes  place  largely  within  the 
"  manifold. 

To  some  considerable  extent  this  process  is  one  of  surface 
carburetion.  The  surface  carbureter  was  abandoned  early  in 
the  art.  Its  faults  are  too  well  known  to  need  further  discussion 
at  this  time,  and  reversion  to  the  functions  of  this  abandoned 
device,  which  has  been  unconsciously  thrust  upon  us  by  the 
present  low  grade  of  fuel,  is  a  curious  coincidence.  Such  con- 
ditions seem  unavoidable,  however,  and  must  be  frankly 
met. 


THE   INTAKE   MANIFOLD  25 


HEATING 

For  several  years  manufacturers  have  provided  means  for 
heating  the  carbureter  by  circulating  water,  while  the  latest 
kerosene  carbureters  employ  the  higher  temperatures  of  the 
exhaust  in  a  jacket  surrounding  the  air-passages. 

Application  of  Heat 

In  view  of  the  fact,  as  determined  herein,  that  a  very  con- 
siderable part  of  the  carburetion  actually  takes  place  after  the 
mixture  has  left  the  carbureter,  it  is  difficult  to  see  why  more 
manufacturers  have  not  followed  the  few  examples  already 
set  them  and  employed  means  for  heating  the  manifold.  Heat 
so  applied  is  most  effectually  communicated  directly  to  the 
deposited  liquid  film,  hastening  evaporation  and  insuring  the 
rapid  diffusion  of  the  fuel  vapor  with  the  entraining  air  in  a 
manner  that  leaves  little  to  be  desired. 

STARTING,  COLD 
Conditions  for  Easy  Starting 

Coming  now  to  the  question  of  practical  manifold  design, 
we  are  at  once  confronted  by  the  starting  period  wherein  no 
heat  is  available.  Without  the  aid  of  heat  the  only  practical 
method  of  securing  comparative  ease  of  starting  is  to  so  design 
the  manifold  that  the  greatest  amount  of  fuel  mist  may  be 
delivered  to  the  cylinders.  As  we  have  seen,  this  entails: 

(a)  Powerful   atomization  in   the   carbureter,   because   the 
smaller  particles  are  more  easily  entrained  at  low  velocities. 

(b)  The  least  possible  manifold  diameter  consistent  with 
volumetric   efficiency   at   subsequent   high   speeds,   insuring   a 
more  thorough  entrainment  of  the  fuel  mist. 

(c)  Smooth   interior    surfaces,    reducing    skin   friction    and 
absence  of  enlargements  of  cross-sectional  area,  as  maintaining 
the  velocity  already  acquired. 

To  these  must  be  added: 

(d)  Minimum  length  of  manifold  passage. 


26  HANDBOOK   OF   CARBURETION 

In  any  event,  the  best  that  can  be  hoped  for  in  starting  cold 
is  that  a  small  portion  of  the  fuel  will  reach  the  cylinders,  either 
in  a  gaseous  or  liquid  form,  sufficient  in  quantity  to  start  the 
cycle.  Hence,  the  utility  of  the  excess  of  fuel  secured  by  "prim- 
ing" and,  incidentally,  the  necessity  of  having  this  priming 
charge  highly  atomized. 

CONTINUOUS  OPERATION 

Let  us  now  consider  the  efficiency  of  such  general  design  in 
delivering  to  the  cylinders  a  truly  gaseous  mixture  after  the 
engine  is  heated. 

Atomization 

(a)  That  fine   atomization  is   a  necessary  prerequisite   is 
evident  when  we  consider  that  the  fuel  particles  are  spherical 
in  shape.     The  volume  or  weight  of  a  sphere  decreases  with 
the  cube  of  its  diameter,  while  the  surface  exposed  to  evaporative 
influences  decreases  only  as  the  square  of  the  diameter.     The 
rapid  increase  of  effective  surface  exposure,  as  diameters  are 
decreased,  is  apparent. 

Area 

(b)  The  proper  diameter  of  the  manifold  is  a  question  for 
the  individual  judgment  of  the  designer.    The  permissible  loss 
of  volumetric  efficiency,  due  to  velocity  head  and  friction  within 
the  manifold,  should  be  adjusted  to  other  factors  of  volumetric 
loss,  such  as  valve  location,  areas,  and  timing.     The  total  loss 
should  be  so  established  that  the  highest  possible  velocities  can 
be  tolerated  within  the  manifold. 

Condition  of  Smoothness 

(c)  As  to  the  choice  between  smooth  and  roughened  interior 
walls,  the  writer  believes,  from  his  experience,  that  with  proper 
heat   distribution   during   continued   operation   there   is   little 
danger  of  unevaporated  fuel  reaching  the  cylinders  with  the 
smoothest  of  interior  walls.     The  numerous  bends,  unavoidable 
in  multi-cylinder  construction,  and  even  the  frictional  opposition 
of  the  conventional  butterfly  throttle,  will  insure  deposition  of 


THE   INTAKE  MANIFOLD  27 

that  portion  of  the  fuel  which  has  escaped  previous  evaporation, 
and,  as  has  been  noted,  the  application  of  heat  to  the  surfaces 
which  receive  this  deposit  will  promote  its  thorough  evaporation. 

Length 

(d)  Consideration  of  the  actual  distance  between  the  car- 
bureter and  the  valve-chambers  shows  a  possibility  of  real 
danger  in  making  the  manifold  too  short.  It  is  conceivable 
that  if  the  foregoing  conditions  are  complied  with,  the  manifold 
might  be  made  so  short  that  unevaporated  liquid  would  actually 
reach  the  cylinders,  resulting  in  inefficient  combustion. 

QUALITATIVE  DISTRIBUTION 

Furthermore,  we  have  already  noted  that  the  mixture 
entering  the  manifold  is  far  from  homogeneous.  To  produce  the 
homogeneity  necessary  for  equal  qualitative  distribution,  we 
must  provide  conditions  favoring  the  rapid  diffusion  of  the  air 
and  fuel  vapor. 

Diffusion 

Just  how  rapid  this  diffusion  must  be  is  best  illustrated  by 
considering  the  time  element  of  the  passage  of  gas  through  the 
manifold.  For  example,  assume  that  the  length  of  the  manifold 
passage  is  two  feet.  At  a  minimum  velocity  of  1,800  feet  per 
minute  a  given  unit  of  gas  remains  in  the  manifold  but  0.066  of  a 
second.  At  the  not  uncommon  velocity  of  8,000  feet  per  minute 
(which  only  entails  a  loss  of  volumetric  efficiency  of  less  than  i  per 
cent),  a  unit  of  gas  remains  in  the  manifold  but  .015  of  a  second. 

Economizers 

Under  these  conditions  a  most  intimate  mixture  of  the  gases 
is  necessary,  and  hence,  the  real  efficiency  of  some  of  the  so-called 
"economizers"  on  the  market.  The  offset,  or  reverse  bends, 
in  the  upright  member  of  some  manifolds,  is  usually  merely  for 
convenience  in  locating  the  carbureter  in  the  limited  space 
available.  The  bends  so  introduced,  if  properly  designed,  are 
not  a  detriment,  as  is  frequently  stated,  but,  instead,  possess  the 


28  HANDBOOK   OF   CARBURETION 

distinct    advantage    of   promoting    diffusion    through    a    more 
thorough  mixing  of  the  gases. 

Qualitative  Distribution 

It  has  been  the  writer's  experience  that  many  faults  of 
operation  were  due  solely  to  uneven  qualitative  distribution  of 
the  mixture.  This  fault,  infrequently  recognized,  results  in  a 
wide  range  of  troubles  from  poor  economy  or  a  slight  lack  of 
power  to  persistent  and  perplexing  missing.  This  being  the 
fact,  the  practice  of  locating  the  carbureter  immediately  at  the 
branchings  of  the  manifold  cannot  be  recommended. 

TYPES  OF  MANIFOLDS 

The  types  of  manifold  shown  in  Figs,  i  and  2  embrace 
this  objectionable  feature.  Similar  designs  are  becoming  more 


FIGS,  i  AND  2. 

common  with  the  adoption  of  pressure  feed  on  the  fuel.  Engines 
so  equipped  are  notably  easy  starting,  but  the  writer  believes 
operative  efficiency  is  sacrificed  as  a  result. 

Fig.  3  shows  the  opposite  extreme  in  an  attempt  to  provide 
diffusion  chambers.  With  highly  volatile  fuel,  or  with  proper 
heating  of  the  vertical  member,  these  chambers  would  doubtless 
afford  distinct  advantages  through  the  mixing  of  the  gases  by 
expansion  and  contraction.  For  cold  weather  starting,  with  the 
fuel  of  the  present  day,  the  writer  has  daily  reason  to  criticize 
this  design. 


THE   INTAKE   MANIFOLD 


29 


A  modification  of  the  diffusion-chamber  idea  is  shown  in 
Fig.  4.  If  constructed  with  the  proper  dimensions  and  with 
the  vertical  member  of  this  manifold  heated,  qualitative  dis- 


FIGS.  3  AND  4. 


FIGS.  5  AND  6. 

tribution  between  the  two  branches  could  hardly  fail  to  be 
excellent.  Starting  cold,  however,  would  be  something  of  a 
problem. 


30 


HANDBOOK   OF   CARBURETION 


Fig.  5  shows  a  type  exhibiting  noticeably  erratic  distribution 
when  used  with  the  unjacketed  carbureter.  With  this  manifold 
of  brass,  with  a  smooth  interior  finish,  the  engine  started  easily 
but  developed  a  noticeable  lack  of  power,  particularly  at  low 
speeds.  An  experimental  manifold  shown  in  Fig.  6  was  con- 
structed of  ordinary  i^-inch  pipe  fittings,  being  practically  the 
same  size  as  the  original  manifold.  In  this  crude  affair  diffusion 
was  secured  by  the  additional  length  of  and  bends  in  the  central 
member,  and  also  in  the  slight  enlargement  of  the  central  tee. 
With  a  highly  atomizing  carbureter  the  smoothness  of  operation 
and  gain  in  power  were  most  marked.  Owing  to  the  difference 
in  the  carbureters  employed,  this  test  is  of  lessened  value  so  far 
as  the  manifold  itself  is  concerned.  It  is  of  value  because  of  the 
close  and  indissoluble  relationship  existing  between  the  work 
of  the  carbureter  and  the  functions  of  the  manifold. 

More  definitely  conclusive  was  the  experiment  performed 
upon  an  engine  equipped  with 
the  manifold  shown  in  Fig.  7. 
With  a  jacketed  carbureter, 
distribution  was  so  poor  in  cold 
weather  as  to  cause  actual  miss- 
ing, which  yielded  to  none  of 
the  usual  remedies,  including 
change  of  carbureters.  Not  only 


FIGS.  7  AND  8. 

was  this  trouble  completely  obviated,  but  marked  increase  in 
power  and  better  general  all-around  action  was  obtained  by  no 
other  change  than  surrounding  the  vertical  member  of  this 


THE   INTAKE   MANIFOLD  31 

manifold  with  a  close-wound  coil  of  five-sixteenth  copper  tubing 
carrying  hot  water  from  the  circulation. 

The  improvement  in  operation  was  so  marked  that  the 
experimental  manifold  shown  in  Fig.  8  was  constructed  with  a 
more  effective  water-jacketing.  Owing  to  its  experimental 
construction  of  brass  pipe  and  standard  fittings,  it  was  im- 
possible to  maintain  the  downward  slope  of  the  branches,  but 
notwithstanding  this,  the  owner  preferred  to  continue  the  use  of 
the  makeshift  rather  than  the  original  manifold.  Of  course, 
the  short  radii  of  the  tee  and  the  elbows  were  indefensible, 
but,  while  starting  cold,  though  not  at  all  bad,  might  have  been 
improved  by  a  permanent  design,  distribution  was  all  that  could 
be  desired. 

QUANTITATIVE  DISTRIBUTION 

From  the  foregoing  it  is  seen  that  the  conditions  required 
for  easy  starting  do  not,  for  the  most  part,  conflict  with  the 
requirements  for  continued  running.  There  remain  to  be 
considered  details  of  design  necessary  to  secure  the  same  quantity 
of  mixture  in  each  cy Under.  Having  made  provisions  to  insure 
a  homogeneous  and  truly  gaseous  mixture,  the  remaining  ques- 
tions simplify  themselves  largely  to  problems  of  equal  frictional 
resistances  in  the  different  branchings. 

Resistance  of  Bends 

Resistance  to  the  flow  of  air  through  pipes  may  be 
readily  determined  from  the  formulae  and  tables  given  in 
the  standard  text-books.  In  computing  this  resistance,  due 
attention  must  be  given  to  the  additional  resistance  offered 
by  bends.  Kent,  8th  edition,  page  593,  gives  a  convenient 
table  on  the  effect  of  bends,  wherein  lengths  of  straight 
pipe,  equivalent  in  resistance  to  bends  of  different  radii,  are 
given.  As  an  illustration  of  the  use  of  this  table  we  note  that 
the  resistance  of  a  standard  1^4  -inch  pipe  elbow  (mean  radius 
i  /32  inch)  is  equivalent  to  a  little  more  than  4  feet  of  straight 
pipe.  If  the  mean  radius  were  increased  to  4/54  inches,  the 
resistance  would  be  reduced  to  that  of  1 1  -Ms  inches  of  straight 


32  HANDBOOK   OF   CARBURETION 

pipe.     By  this  method  the   total  resistance  of  the  branches 
may  be  determined  and  equalized. 

It  must  be  borne  in  mind,  however,  that  bends  are  prolific 
of  deposition  of  entrained  liquid,  and  therefore  the  drainage  of 
these  bends  should  be  carefully  directed  toward  the  heated 
surfaces.  In  furtherance  of  this  idea,  the  branches  should  be 
given  a  drainage  slope  away  from  the  cylinders.  Fig.  3  shows 
that  careful  consideration  has  been  given  to  these  details.  Note 
the  longre  radius  of  the  bend  of  greater  angularity  and  the 
location  of  the  junction  of  the  upright  member  to  the  right  of 
the  center.  Note  also  the  downward  slope  of  the  lower  surfaces 
of  the  cross  members  toward  the  upright  member.  All  these 
details  tend  to  equalize  distribution,  both  quantitative  and 
qualitative. 

SIX-CYLINDER  DISTRIBUTION 

Six-cylinder  engines  present  greater  complications  in  the 
matter  of  quantitative  distribution  than  do  the  fours.  In 
fact,  the  development  of  the  early  sixes  was  retarded  by  a  lack 


FIGS.  9  AND  10. 

of  understanding  of  actual  conditions  within  the  manifold.  The 
greater  distances  to  be  travelled  by  the  gases,  the  more  numer- 
ous branchings  and  the  overlapping  of  the  suction  strokes,  all 
emphasize  the  tendencies  toward  uneven  distribution. 

Fig.  9  shows  one  of  the  methods  employed  to  obviate  this 


THE   INTAKE   MANIFOLD  33 

difficulty.  In  this  manifold  it  will  be  noted  that  the  supply 
to  each  cylinder  is  drawn  from  both  branches.  As  the  resistance 
is  increased  by  the  greater  distance  travelled  in  one  branch, 
it  is  proportionately  decreased  by  the  shorter  distance  travelled 
in  the  other  branch,  and  hence  is  constant. 

Another  arrangement  giving  the  same  effect  is  a  horizontal 
pipe  carrying  a  longitudinal  partition,  or  baffle  plate,  as  an  integral 
part  of  the  casting. 

The  same  result  is  sought  by  a  different  arrangement,  shown 
in  Fig.  10. 

CONCLUSIONS 

The  conditions  outlined  in  this  chapter  are  fundamental. 
They  can  be  met  in  a  variety  of  ways  which  will  suggest  them- 


FlGS.    II    AND    12. 

selves  to  the  designer.     Briefly  summarized  these  conditions 
consist  of: 

1.  A  heated  manifold. 

2.  High  manifold  velocities. 

3.  Smooth  interior  walls. 

4.  Long  radius  curves. 

5.  No  enlargement  of  cross-sectional  area. 

6.  Absence  of  liquid  retaining  pockets. 

7.  Branches  sloping  toward  the  central  member. 

8.  Adequate  provision  for  mixing  the  gases. 

9.  Equal  resistance  in  the  branches. 

The  writer  has  been  unable  to  secure  an  illustration  of  a 
manifold  embracing  all  these  features  if,  indeed,  such  exists*. 


34  HANDBOOK  OF   CARBURETION 

The  general  idea  is  expressed  in  Fig.  n,  which,  in  point  of 
fact,  is  a  manifold  of  one  of  the  best  known  cup-winning  cars. 
Improvement  might  be  made  in  this  design  by  a  slight  drainage 
slope  given  to  the  branches  and  by  water-jacketing  the  central 
member.  It  is,  of  course,  assumed  that  the  diameter  of  this 
manifold  is  properly  proportioned  to  the  displacement  of  the 
engine. 

Fig.  12  embraces  every  apparent  fault  that  can  be  introduced 
into  a  manifold.  Its  diameter  is  great.  Its  bends  are  sharp. 
Drainage  is  directly  away  from  the  central  member.  Pockets 
are  formed  at  the  base  of  the  branches.  It  has  no  provision  for 
diffusion,  is  unheated,  and,  if  in  consonance  with  the  rest  of  the 
design,  its  interior  walls  are  doubtless  rough. 


CHAPTER  III 
CARBURETER  TESTING 

On  the  Block 

THE  usual  test  to  which  a  carbureter  is  subjected  while 
attached  to  an  engine  on  the  block  consists  of: 

(1)  A  series  of  readings  of  maximum  horse-power  at  various 
speeds.    With  the  throttle  wide  open,  various  loads  are  imposed 
upon  the  engine  and  the  resulting  horse-power  curve  plotted 
therefrom.    The  fuel  consumption  is  also  noted  at  each  speed 
and  the  resulting  curve  plotted. 

(2)  This  programme  is  sometimes  elaborated  by  a  series  of 
runs  at  three-fourths,  one-half,  and  one-quarter  throttle  with  the 
results  given  expressing  horse-power  developed  and  the  fuel  used. 

(3)  More  infrequently,  the  rate  of  acceleration  is  noted  as 
the  number  of  seconds  required  to  reach  a  given  speed,  either 
running  light  or  with  some  empirical  load. 

(4)  Very  rarely  flexibility  is  determined  by  a  mechanical 
device  which  slowly  closes  the  throttle  and  then  suddenly  opens 
it,  and  then  reverses  its  operation  by  opening  the  throttle  slowly 
and  snapping  it  shut. 

Maximum  Horse-Power 

Determination  of  the  maximum  horse-power  curve  (i)  is,  of 
course,  an  essential  detail  of  any  carbureter  test.  It  shows  any 
erratic  behavior  in  the  functioning  of  the  instrument  and  detects 
any  undue  internal  resistance. 

For  motor-boat  requirements,  where  maximum  horse-power 
at  full  speed  is  of  primary  importance,  this  test  gives  the  most 
desired  information. 

In  automobile  practice,  however,  it  is  a  rare  occurrence  that 
an  engine  is  called  upon  to  deliver  its  maximum  power  at  its 
highest  speed,  except  in  the  case  of  racing  cars.  Nine-tenths 
of  all  driving  is  done  with  the  throttle  partially  closed,  and 

35 


36  HANDBOOK   OF   CARBURETION 

consequently  the  object  of  the  test  on  the  block  should  be  to 
determine  the  relative  performance  of  carbureters  under  various 
throttle  openings. 

A  comparison  of  carbureter  tests  conducted  for  maximum 
horse-power  alone  will  disclose  surprisingly  little  difference 
either  in  power  developed  or  in  fuel  consumption.  The  same 
carbureters  will,  however,  show  markedly  different  results  under 
road  conditions. 

In  chapters  I  and  II  the  causes  for  these  different  perform- 
ances have  been  analyzed.  To  actually  determine  the  relative 
merits  of  various  devices  on  the  block,  it  is  necessary  to  simulate 
road  conditions  in  so  far  as  it  is  possible  to  do  so.  The  first 
of  these  conditions  to  be  observed  is  that  exhaustive  tests 
must  be  conducted  at  different  throttle  openings. 

Fallacy  of  Set  Throttle  Tests 

If  comparisons  are  to  be  accurately  made,  the  plan  usually 
followed  as  outlined  in  (2)  is  fallacious,  because,  owing  to  lack 
of  any  standardization  of  throttle  sizes,  shapes,  or  even  types, 
the  same  position  of  the  throttle  arm  or  crank  does  not  neces- 
sarily mean  equal,  or  even  approximately  equal,  openings  on  any 
two  instruments.  Nor  is  a  car  driven  with  any  reference  to, 
indeed  seldom  with  knowledge  of,  the  amount  of  throttle  opening 
afforded  by  intermediate  positions  of  the  throttle  lever  on  the 
steering-wheel.  Instead,  the  throttle  is  opened  until  a  certain 
result  is  accomplished,  i.e.,  the  moving  of  a  given  load  at  a  given 
speed. 

Testing  With  Fixed  Load 

To  reproduce  this  condition  on  the  block,  a  certain  load 
should  be  set  off  on  the  dynamometer  scale  and  the  throttle 
opened  until  the  beam  balances  at  the  desired  speed.  With 
the  electric  dynamometer  the  load  increases  automatically  with 
the  speed.  This  requires  simultaneous  adjustment  of  both 
rheostat  and  carbureter  throttle.  With  the  hydraulic  dyna- 
mometer a  somewhat  similar  condition  exists,  necessitating 
simultaneous  regulation  of  hand-wheel  on  the  brake  and  throttle 


CARBURETER  TESTING  37 

of  the  carbureter.  These  adjustments  are,  however,  easily 
made  after  a  little  practice. 

By  repeating  the  foregoing  with  various  loads  carried  through- 
out a  range  of  speeds,  points  may  be  determined  from  which 
may  be  plotted  a  curve  fairly  representative  of  "Part  Throttle 
Performance."  This  curve  will  prove  of  far  more  value  in 
comparing  the  performance  of  different  instruments  than  will 
the  maximum  horse-power  curve  alone. 

In  order  to  determine  the  true  characteristics  of  the  curve, 
the  following  procedure  is  recommended.  Having  determined 
the  maximum  torque  of  the  engine,  this  load  is  divided  into 
equal  parts — say  fifths.  Runs  are  then  made  throughout  the 
entire  speed  range,  with  the  motor  carrying,  say,  one-fifth  of 
its  maximum  load.  Limits  of  speed,  both  slow  and  fast,  are 
noted,  together  with  the  fuel  consumption.  This  test  is  repeated 
for  two-fifths,  three-fifths,  and  four-fifths  load. 

The  results  obtained  are  frequently  surprising.  A  carbureter 
that  will  show  excellent  economy  on  full  throttle  may  fail 
utterly  to  carry  a  given  load  at  a  certain  speed  on  part  throttle, 
necessitating  an  enrichment  of  the  mixture  that  will  show  the 
futility  of  the  record  established  at  full  load. 

The  fuel  consumption  curves  may  be  comparatively  smooth 
at  full  throttle  but  widely  variant  while  carrying  constant  load 
throughout  the  speed  range.  Speed  limits,  both  high  and  low, 
will  be  found  to  vary  greatly  with  different  types  of  carbureters. 

Under  this  method  of  testing,  the  different  types  discussed 
in  Chapter  I  never  fail  to  exhibit  all  the  peculiarities  enumerated 
therein. 

Acceleration 

In  addition  to  the  foregoing,  acceleration  (3)  should  be 
determined  as  outlined,  but  this  determination  should  be  made, 
when  possible,  with  all  the  different  loads  mentioned  in  the 
preceding  test.  In  this  connection,  however,  it  is  well  to  note 
that  the  automatically  increasing  load  of  the  electric  or  hydraulic 
dynamometer  is  unobjectionable  for  the  purpose  of  determining 
acceleration,  as  it  closely  simulates  road  conditions  where  the 

51936 


38  HANDBOOK  OF  CARBURETION 

load  increases  by  wind  resistance  approximately  as  the  square 
of  the  speed. 

Flexibility 

The  test  outlined  in  (4)  is  of  great  practical  merit  for  purposes 
of  comparison,  if  properly  conducted.  If  the  variable  throttle 
moving  device  is  mechanically  operated  so  its  movements  may 
be  continued  over  a  considerable  time-period,  it  is  frequently 
found  that  after  several  repetitions  some  carbureters  will  choke, 
even  though  this  tendency  may  not  be  in  evidence  during  one  or 
two  trials.  This  may  be  due  to  undue  enlargement  of  area  and 
consequent  reduction  of  velocity.  Whatever  its  cause,  it  is  a 
prolific  source  of  annoyance  when  driving  a  car  through  traffic, 
and  should  be  detected  by  a  properly  conducted  block  test. 

Like  the  preceding  tests  (i  to  3),  test  (4)  should  be  con- 
ducted at  various  loads,  for  it  will  be  found  that,  as  in  the  other 
instances,  performance  will  usually  vary  widely  at  different 
loads. 

Again,  if  this  flexibility  test  is  of  sufficiently  long  duration, 
the  fuel  consumption  may  be  accurately  measured.  A  determi- 
nation of  this  kind  gives  a  far  more  accurate  measure  of  the 
actual  performance  of  a  carbureter  in  practical  road  use  than  is 
obtainable  by  any  other  system  of  averages. 

In  city  use  particularly,  a  car  is  rarely  driven  two  consecutive 
minutes  with  the  same  throttle  setting,  and-  consequently,  as  is 
well  known,  fuel  consumption  is  much  higher  than  in  the  case 
of  a  cross-country  run,  which  may  be,  in  some  measure,  com- 
pared to  a  constant  load  in  block  testing. 

With  a  full  block  test,  conducted  along  the  lines  herein  out- 
lined, no  function  of  the  carbureter  will  escape  scrutiny.  Com- 
parisons of  different  types  will  serve  to  establish  their  relative 
merits  and  their  peculiar  adaptability  to  the  engine  used  in 
the  test. 

Practical  Results 

For  automobile  use,  practical  interest  centres  chiefly  in  part- 
throttle  performance,  acceleration,  and  flexibility.  Economy 
is  of  course  desirable,  but  becomes  of  primary  importance  only 


CARBURETER   TESTING  39 

in  the  larger  units,  while  maximum  power  at  maximum  speed 
is  a  consideration  confined  wholly  to  racing  cars. 

In  marine  practice,  the  demand  for  maximum  power  from  a 
given  size  of  engine,  coupled  closely  with  minimum  fuel  con- 
sumption, is  the  chief  consideration,  followed  closely  by  a  de- 
mand for  maximum  speed.  Part-throttle  performance  is  of 
less  importance,  while  flexibility  and  acceleration  are  the  last 
consideration. 

Automatic  Apparatus 

In  the  testing  laboratory,  measurements  should  be  made  as 
automatic  as  possible.  A  convenient  method  of  accomplishing 
this  result  is  to  have  the  fuel  tank  balanced  on  a  pair  of  scales. 
The  beam  of  these  scales  in  falling  closes  an  electric  circuit 
which  starts  a  stop-watch,  revolution  counter,  and  bell.  When 
the  bell  sounds,  the  operator  reduces  the  weight  on  the  scale 
beam  by  one  pound  (or  whatever  other  unit  seems  desirable). 
When  this  unit  is  consumed,  the  beam  falls  again,  closing  the 
circuit.  This  disengages  the  revolution  counter  and  stops  the 
stop-watch,  while  the  bell  announces  the  end  of  the  run. 

CARBURETER  TESTING  ON  THE  ROAD 

Block  Testing  Insufficient 

No  matter  how  comprehensive  a  block  test  may  be,  the 
practical  man  bases  his  final  judgment  of  the  merits  of  a  car- 
bureter by  its  actual  performance  on  the  car.  This  is  wisdom 
born  of  experience.  Though  we  may  simulate  certain  road 
conditions  on  the  block,  there  are  certain  factors  encountered 
in  road  work  that  cannot  be  duplicated;  for  example,  the  load 
on  the  motor  increasing  with  the  square  of  the  speed,  the  loss 
in  transmission  from  motor  to  the  rear  wheels.  Then  tire 
losses,  and  even  rolling  resistance  of  the  car  itself  varies  not  only 
with  every  make,  but  with  every  changing  condition  of  roadway 
itself,  or  with  the  whims  of  the  wind. 

Difficulties  in  Comparing  Results 

Vibration,  road  shock,  changes  of  float-tank  level  from 
gradients  encountered,  and  sometimes  sudden  changes  of  temper- 


40 


HANDBOOK   OF    CARBURETION 


ature,  and  finally  dust  and  dirt,  are  among  the  road  conditions 
which  a  carbureter  must  faithfully  meet,  and  which  cannot  be 
reproduced  in  any  test.  Small  wonder  that  experience  has 
taught  us  to  look  askance  at  any  test  which  necessarily  omits 
these  factors  which  must  be  met  daily.  At  the  same  time, 


CE  OF  ROAD  AND 
ADJUST  THE  LEVELLING  SCREW  UNTIL  NEEDLE  POINTS  TO  ZERO. 


HE   UPPER  SCALE  RECORDS  ACCELERATION  DUE  TO  TRACTIVE  EFFORT  OF 
ENGINE  OR  RETARDATION  DUE  TO  ROAD  RESISTANCE  OR  OTHER  FRICTIONA' 
FORCES.      WHEN  COASTING,  THE  RETARDATION  READING  EQUALS 
POUNDS  PER  TON. 


FIG.  13.    THE  ACCELEROMETER. 

when  we  scan  the  array  of  formidable  conditions,  we  are  less 
likely  to  place  too  much  dependence  in  the  opinion  of  any 
individual  on  the  comparative  road  performance  of  competing 
devices.  Two  carbureters  may  be  tried  on  the  same  car  and 
for  the  same  distance  over  the  same  road,  but  speeds  cannot  be 
maintained  the  same  at  every  point. 

Barometric  and  temperature  changes  cannot  be  accurately 
compared.     Windage  may  not  be  the  same  during  both  tests. 


CARBURETER   TESTING  41 

The  driver  in  each  instance  will  not  press  his  accelerator  the 
same  amount  at  the  same  place.  In  brief,  conditions  cannot 
even  approximate  constancy  in  both  trials. 

Comparative  road  testing  can,  therefore,  be  of  value  only 
when  each  test  is  conducted  over  a  period  sufficiently  long  to 
minimize  the  errors  by  the  law  of  averages.  This  requirement 
is  one  not  always  easily  fulfilled,  and  therefore  it  would  seem 
desirable  to  find  a  method  of  car  testing  which  will  give  an 
accurate  comparison  of  performance  of  various  devices,  either 
actually  on  the  road  or  under  controllable  conditions  as  closely 
approaching  those  of  the  road  as  is  possible  within  the  confines 
of  a  laboratory. 

THE  ACCELEROMETER 

This  instrument  was  designed  by  H.  E.  Wimperis,  M.A., 
A.M.I.C.E.,  A.M.I.E.E.,  of  England.  The  outward  appearance 
of  the  instrument  is  shown  in  Fig.  13,  while  the  construction 
is  shown  in  Fig.  14. 

The  instrument  has  no  mechanical  connection  with  the  car. 
It  is  simply  carried  in  any  convenient  position  on  the  car  where 
it  can  be  leveled  by  means  of  the  adjusting  screws  on  its  base. 

The  dial  of  the  instrument  carries  a  double  scale,  reading 
each  way  from  o.  The  upper  scale  reads,  "Acceleration  in 
feet  per  second  per  second"  on  one  side  of  o,  and  "Retardation 
in  Ibs.  per  ton  of  2,000  Ibs."  on  the  other  side.  The  lower  scales 
read  "Upward  Gradient,"  and  "Downward  Gradient." 

DESCRIPTION 
Principle  of  Operation 

The  instrument  depends  for  its  operation  on  the  inertia  of  a 
copper  weight  A  (Fig.  14).  The  centre  of  gravity  of  this  weight 
is  eccentric  to  its  axis  of  revolution  B.  Any  force  in  the  direction 
of  the  arrow  on  the  dial  tends  to  make  the  mass  of  copper  lag. 
This  lag  rotates  the  spindle  B,  which,  by  means  of  the  gear  train 
C,  rotates  the  spindle  D,  winding  up  the  hair-spring  E. 

The  hair-spring  is  so  calibrated  that  the  pointer  reads  ac- 


42 


HANDBOOK   OF    CARBURETION 


curately  on  the  dial  scales  the  actual  forces  affecting  the  copper 
disk. 

Compensation 

Any  tendency  of  the  disk  to  oscillate  is  checked  by  its  passage 
between  the  poles  of  the  permanent  magnet  F.    The  arrange- 


FlG.    14.      ACCELEROMETER   SECTION. 

ment  of  the  gearing  effects  what  is  called  a  "compensating 
balance"  which  neutralizes  transverse  forces  and  causes  the 
instrument  to  record  correctly  even  on  heavily  cambered  roads. 
The  reading  of  the  instrument  is  in  no  way  affected  by  grade, 
as  will  be  seen  by  consideration  of  its  principle  of  operation. 


CARBURETER   TESTING  48 

Let  F  =  Force  in  pounds. 

W 

M  =  Mass  or  — 
g 

A  =  Acceleration  in  feet  per  second  per  second. 
g  =  Acceleration  due  to  gravity,  32.16  feet  per  second 

per  second  in  middle  latitudes. 
W  =  Weight  in  pounds. 
G  =  Gradient  in  percentage. 
Then  as 

F  =  M  A  (17) 


The  force  necessary  to  move  a  unit  weight  up  a  grade  is 

WG 

F  = (10) 

100 

Substituting  the  value  for  F  in  equation  (18),  the  acceleration 
equivalent  to  this  force  is 

32.2  WG 

100  W 
which  reduces  to 

A  =  0.322  G  (20) 

Assume  that  a  car  weighs  i  ton  and  is  equipped  with  an 
engine  that  will  produce  an  accelerative  force  of  2  feet  per  second 
per  second  on  a  level. 

.Acceleration  Up-Grade 

Suppose  now  that  this  car  is  ascending  a  2  per  cent  upward 
gradient,  which  graduation  is  coincident  with  0.624  m  the  ac- 
celeration scale.  [See  equation  (20).]  The  engine  is  therefore 
exerting  a  force  equivalent  to  an  acceleration  of  0.624  ft./sec./sec. 
in  maintaining  constant  speed.  When  the  throttle  is  wide 
opened  the  speed  of  the  car  will  increase  and  the  needle  will 
stand  at  2  ft./sec./sec.  That  is,  the  acceleration  from  the 
initial  speed  and  on  the  2  per  cent  grade  will  be  (2  —  .624)  = 


44  HANDBOOK   OF   CARBURETION 

1.376  ft./sec./sec.,  but  the  total  accelerative  power  will  be  2, 
as  it  was  on  the  level. 

Acceleration  Down-Grade 

Or  assume  this  car  to  be  descending  a  3  per  cent  gradient. 
The  force  of  gravity  urging  the  car  forward  will  be  0.966  ft./ 
sec./sec.  Hence,  upon  open  throttle  the  total  force  moving 
the  car  forward  will  be  0.966  +  2  =  2.966  ft./sec./sec.,  and  the 
needle  will  swing  through  this  arc,  but  the  0.966  being  on  the 
opposite  side  of  o,  the  needle  will  again  stand  at  2  ft./sec./sec. 
on  the  acceleration  scale. 

It  is  thus  seen  that  acceleration  can  be  measured,  irrespective 
of  grade,  by  suddenly  opening  the  throttle  wide  and  noting 
the  reading  on  the  acceleration  scale. 

Retardation  is  similarly  read  on  the  opposite  scale  and  is 
subject  to  the  same  compensation  as  regards  grade.  Thus,  by 
equation  (19)  the  force  acting  in  the  opposite  direction  to  the 
motion  of  the  one-ton  car  on  a  2  per  cent  gradient  would  be 

2OOO  X    2 

F  =  —  =  40  pounds  per  ton. 

100 

Retardation  Up-Grade 

If  the  car  were  ascending  a  2  per  cent  grade  at  constant 
speed  and  the  power  were  suddenly  shut  off,  the  needle  would  re- 
turn to  o,  provided  the  car  had  no  rolling  resistance.  As  a  matter 
of  fact,  the  needle  swings  to  the  right  of  o,  an  amount  which 
consequently  registers  the  resistance  of  the  car  in  pounds  per 
ton  upon  the  upper  or  retardation  scale. 

Retardation  Doimi-Grade 

On  the  other  hand,  consider  the  car  as  being  driven  down  a 
2  per  cent  grade  at  constant  speed,  and  the  power  suddenly 
discontinued.  If  the  rolling  resistance  was  greater  than  40 
pounds  per  ton  (as  it  must  be  to  necessitate  the  use  of  power),  a 
preponderance  of  force  would  be  exerted  in  a  direction  opposed 
to  forward  motion  and  the  lag  of  the  copper  disk  would  cause 
the  needle  to  move  further  to  the  right  by  an  amount  which, 


CARBURETER   TESTING  45 

minus  the  gradient  reading,  would  be  a  true  measure  of  the 
preponderance  of  retarding  force,  while  the  needle  will  give  a 
direct  reading  of  the  total  rolling  resistance  on  the  retarda- 
tion scale,  as  in  the  previous  instance. 

A  thorough  understanding  of  these  functions  of  the  instru- 
ment makes  its  practical  use  easy. 

DETERMINATION  OF  RESISTANCE 

For  the  determination  of  rolling  resistance,  procedure  is  as 
follows:  The  a.ccelerometer  is  placed  on  the  car,  with  the 
arrow  on  the  dial  pointing  in  the  direction  of  motion.  It  is 
carefully  levelled,  by  means  of  the  adjusting  screw  at  its  base, 
until  the  needle  stands  at  o,  when  the  car  is  standing  on  the 
level.  If  the  car-body  is  subject  to  much  vibration,  the  instru- 
ment should  be  secured  in  this  position  by  means  of  proper 
straps  and  its  level  position  should  be  checked  as  frequently  as 
possible. 

Method  of  Reading 

The  car  is  next  driven  on  the  high  gear  at  some  constant 
speed,  say  10  miles  per  hour,  preferably  down  a  slight  grade. 
The  clutch  is  suddenly  thrown  out  of  engagement  and  the 
reading  taken  on  the  retardation  scale  before  the  car  speed  changes. 
Owing  to  the  momentum  acquired  by  the  copper  disk,  the  first 
swing  of  the  needle  is  to  be  disregarded,  but  it  will  be  found  to 
speedily  settle  on  the  true  reading.  A  little  practice  will  make 
this  point  readily  determinable. 

Total  Resistance 

A  series  of  readings  taken  at  various  speeds  will  give  the 
curve  (R)  of  rolling  resistance  in  pounds  per  ton,  which  may  be 
readily  reduced  to  total  rolling  resistance  by  multiplying  by 
the  total  weight  in  tons. 

Engine  Friction 

A  similar  curve  (7)  may  be  prepared  by  switching  off  the 
ignition  instead  of  declutching.  The  difference  between  curves 
(/)  and  (R)  will  be  the  friction  of  the  engine. 


46 


HANDBOOK  OF  CARBURETION 


Transmission  Friction 

A  third  curve,  N,  is  obtained  by  throwing   the  gear-shift 
lever  into  the  neutral  position.      Curve  R  —  curve  N  =  friction 


130 
120 
110 
100 
d90 

I» 

L 

I60 

50 
40 
30 
20 
10 
0 

/ 

/ 

/ 

1 

I 

/ 

/ 

/ 

/ 

/ 

y 

I 

/ 

V 

1 

f 

/ 

V 

/ 

/ 

/ 

V 

/ 

/ 

/ 

/ 

/ 

/ 

7 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

r 

^/ 

/ 

X 

x 

X 

^ 

x 

, 

2 

/ 

^-> 

^ 

_F_ 

.  —  • 

—  - 

^ 

s 

C^ 

-^ 

5          10          15 

Miles  per  Hour 

CHART  II. 


25         30 


in  the  transmission,  including  the  effect  of  the  transmission 
brake,  if  any  is  used. 

These  curves  are  plotted  in  Chart  II, 


CARBURETER  TESTING  47 

/  is  the  resistance  with  ignition  off. 

R  is  the  resistance  declutched. 

N  is  the  resistance  with  gears  in  neutral. 

F  is  the  engine  friction,  or  I-R. 

C  is  the  friction  in  transmission,  or  R-N. 

Location  of  Mechanical  Defects 

These  curves  form  the  basis  of  all  subsequent  calculations. 
They  are  useful  also  in  detecting  and  locating  mechanical  defects 
in  the  mechanism  of  the  car.  The  curve  R  may  be  determined 
with  either  set  of  gears  in  mesh,  and  the  friction  of  each  be  thus 
determined.  By  this  method  the  cause  of  a  decrease  of  power 
may  be  located,  in  worn  bearings  or  gears,  sprung  shafts,  in- 
sufficient lubrication,  or  dragging  brake  bands. 

DETERMINATION  OF  ACCELERATION 

Having  found  the  rolling  resistance  of  the  car,  the  next  step 
is  to  determine  acceleration.  This  is  done  by  driving  the  car 
at  a  given  speed  and  suddenly  opening  the  throttle  wide.  Ac- 
celeration is  then  read  directly  from  the  acceleration  scale. 
As  in  the  case  of  retardation,  acceleration  readings  are  taken 
throughout  as  wide  a  speed  range  as  possible  and  a  curve  plotted. 

?2.2  F 
By  equation  (18),  A  =  *    '-^   ,  therefore 

AW 


Hence,  the  force  exerted  by  an  engine  giving  an  acceleration  of 
A  to  a  car  weighing  one  ton  will  be 

2000  A 
FI  =  -^~  =  62.2A  (22) 

Draw-Bar  Pull 

This  is  the  force  over  and  above  that  necessary  to  overcome 
the  rolling  resistance  R.    The  draw-bar  pull  P  per  ton  is  therefore 

P  =  62.2A+R  '(23) 


HANDBOOK   OF   CARBURETION 


Brake  Horse-Power 

The  brake  horse-power  exerted  at  the  clutch  may  be  deter- 
mined as  follows: 

Miles  per  hour  =  ^'2  °=  88  feet  per  minute 
oo 

88  PSW 
and 


where 

BHP  =  Brake  horse-power. 

S  =  Speed  in  miles  per  hour. 
W  =  Weight  of  car  in  tons. 

Indicated  Horse-Power 

The  indicated  horse-power  (IH  P)  may  be  found  by  sub- 
stituting the  values  of  points  on  curve  /  (ignition  off)  for  those 
of  R  in  equation  (23). 

Chart  III  shows  the  resistance  curves  7  and  R',  the 
acceleration;  the  draw-bar  pull;  and  the  indicated  and  brake 
horse-power  of  a  car  weighing  1.57  tons,  equipped  with  a  four- 
cylinder  engine,  4  x  4^  inches;  gear  ratio,  3.5  on  direct  drive. 
Wheels,  33  inches  diameter. 

BASES  FOR  COMPARATIVE  PERFORMANCES 
Brake  Mean  E/ective  Pressure 

A  mathematical  basis  for  comparing  engine  performances  is 
afforded  by  determining  the  "brake  mean  effective  power." 
Let 

7?  77  P 
77  =  Mechanical  efficiency  =  ~rpTp 

p  =  Indicated  mean  effective  pressure  in  Ibs./sq.  inch, 


CARBURETER   TESTING 


then     rip  =  Brake  mean  effective  pressure  in  Ibs./sq.  inch; 
now  if    g  =  gear  ratio, 

D  =  Displacement  of  the  cylinders  in  cubic  inches, 


200  2.0 
180  1.8 
160   1.6 
140  -o  1.4 
£  120  f  1.2 

a  a 

1  100  1  1.0 

1  a 

80  £0.8 
60  0.6 
40  0.4 

\ 

30 
25 

20  g 

W 
10 

5 
( 

s 

^ 

\^ 

\ 

\ 

>\. 

^__ 

\ 



. 

-  — 

•  . 

^s 

—  — 
\ 

^  —  : 
P 

^ 

= 

^ 

/ 

x 

r 

\ 

j 

/ 

X 

. 

/ 

V 

/ 

' 

^ 

/ 

/ 

x^^ 

X 

V 

V 

/ 

^ 

v 

X 

x 

\ 

V  X 

X" 

^ 

x 

^J^;, 

x 

x 

•\ 

X 

^ 

. 

s 

\ 

x 

x 

^ 

/ 

\ 

^ 

X 

x 

i^ 

X 

^ 

\ 

x 

-X 

\ 

^*-> 

—  • 

X 

x 

V 

20  0.2 

/ 

xl 

\ 

X 

0   0 

\ 

5     10         20         30         40         5( 

Miles  per  Hour 
CHART  III. 


d  =  diameter  of  drive  wheels  in  inches, 
P  =  Total  draw-bar  pull  in  pounds, 


then 

and  consequently 


(25) 


THERMAL  EFFICIENCY 

The  accelerometer  furnishes  a  means  for  directly  determining 
the  thermal  efficiency  of  the  engine  by  means  of  the  resistance 


50  HANDBOOK   OF   CARBURETION 

curve  (R)  coupled  with  the  actual  mileage  obtained  on  a  known 
quantity  of  gasoline. 

The  foot-pounds  of  work  performed  are 

R  X  5280  X  M 
when 

R  =  Average  resistance  of  the  run. 
M  =  Miles  per  gallon  of  gasoline. 
s  =  Specific  gravity  of  the  gasoline. 
H  =  Its  heat  value  in  B.T.U.  per  pound. 
Then  the  thermal  efficiency  of  the  engine  will  be 

R  X  M  X  5,280 

8.3455  X  778 # 
which  reduces  to 

•81326  ygr  (26) 

A  more  convenient  formula  of  sufficient  accuracy  for  most 
tests  is 

7p    i  <r 

Thermal  efficiency  =   -^ (27) 

This  formula  assumes  gasoline  of  0.72  specific  gravity  and  a  heat 
value  of  about  20,500  B.T.U.  per  pound. 

GENERAL  OBSERVATIONS 
Accuracy 

Practice  with  the  accelerometer  will  lead  to  a  surprising 
degree  of  accuracy  in  the  results  obtained.  Two  separate 
observers  with  different  instruments  have  obtained  results 
from  the  same  car  varying  less  than  5  per  cent. 

It  is,  however,  necessary  to  accept  only  the  mean  of  many 
readings.  The  instrument  is  not  as  sensitive  as  would  seem 
desirable,  and  apparently  might  be  equipped  with  jeweled  bear- 
ings to  advantage.  It  should  also  be  provided  with  means  for 
holding  it  securely  to  the  car-body. 


CARBURETER   TESTING  51 

Levelling 

Especially  should  great  care  be  exercised  in  its  initial  levelling. 
This  necessitates  the  selection  of  a  perfectly  level  spot.  The 
needle  should  then  be  swung  each  way  from  o  until  it  invariably 
comes  to  rest  on  the  o  mark.  This  adjustment  should  be 
repeated  as  often  during  the  test  as  conditions  will  allow. 

E/ect  of  Wind 

It  is,  of  course,  apparent  that  the  force  and  direction  of  the 
wind  will  materially  affect  the  results  obtained,  hence  it  is  desir- 
able to  select  either  a  still  day,  or  a  road  at  right  angles  to  the 
direction  of  the  wind. 

These  practical  difficulties  have  made  necessary  some  method 
of  reproducing  road  conditions  in  the  laboratory.  Such  a 
method  was  proposed  by  the  author  and  Prof.  E.  H.  Lockwood, 
of  the  Sheffield  Scientific  School  of  Yale  University.  The  follow- 
ing is  from  a  paper  prepared  by  them  for  the  Society  of  Auto- 
mobile Engineers.  The  method  furnishes  such  an  excellent 
means  for  carbureter  testing  that  it  is  quoted  here  complete. 


CHAPTER  IV 

THE  PRACTICAL  TESTING  OF  MOTOR-VEHICLES 

MOTOR-CAR  testing  should  be  conducted  for  two  purposes: 

First. — To  determine  the  actual  performance  of  the  car  as 
a  whole. 

Second. — To  determine  the  relative  merits  of  the  different 
components  of  the  car. 

The  first  is  of  practical  interest  to  the  sales  department,  the 
owner,  and  the  general  public.  Interest  in  the  second  is  con- 
fined largely  to  the  department  of  engineering.  But  from  the 
engineer's  standpoint,  much  useless  experimenting  could  be 
avoided  by  an  accurate  knowledge  of  the  relative  performance 
of  different  motor-vehicles,  as  at  present  designed,  before  at- 
tempting any  comparison  of  constructional  details. 

ROAD  TESTING 

Any  attempt  at  determining  the  actual  performance  of  a  car 
on  the  road  is  confronted  with  the  problem  of  the  uncontrollable 
variables  introduced.  Chief  among  these  are  the  following: 

(a)  Condition  of  roadway. 

(b)  Force  and  direction  of  the  wind. 

(c)  Frequent  and  uncertain  change  of  gradients. 
Among  the  instrumental  difficulties  encountered  are: 

(d)  Lack  of  accurate  apparatus  for  the  determination  of 
power,  without  a  specially  constructed  car. 

(e)  Inability  to  measure  fuel  consumption  accurately,  owing 
to  the  vibration  of  the  car-body. 

PROPOSED  TESTS  OF  PERFORMANCE 

The  purpose  of  this  paper  is  to  suggest  a  method  of  testing 
actual  car  performance  on  the  block  with  results  that  can  be 
reproduced  on  the  road. 

52 


PRACTICAL   TESTING   OF   MOTOR-VEHICLES 


53 


The  method  has  been  developed,  and  is  at  present  employed 
in  the  Mason  Laboratory  of  Mechanical  Engineering  of  the 
Sheffield  Scientific  School,  Yale  University. 


FIG.  15.    TAKING  ROLLING  RESISTANCE 


FIG.  1 6.    CAR  ON  TEST  STAND. 


The  testing  apparatus  is  located  on  the  ground  level  near  the 
Temple  Street  entrance  to  the  laboratory.     An  open  stretch  of 


54  HANDBOOK    OF    CARBURETION 

level  granolithic  concrete  floor,  about  75  feet  long,  permits  of 
towing  tests  to  determine  rolling  resistance  of  the  car  at  low  speeds. 
For  power  tests  the  car  is  placed  on  traction  drums  where  ap- 
pliances are  at  hand  to  measure  power  and  pull  at  different 
speeds.  The  general  appearance  of  the  car  undergoing  various 
tests  is  shown  in  Figs.  15  and  16. 

ROLLING  RESISTANCE 

The  first  test  is  to  determine  the  force  required  to  pull  the 
car  slowly  on  the  smooth  level  floor  of  the  laboratory.  This  is 
accomplished  by  a  recording  dynamometer  attached  to  the  front 
of  the  car,  as  shown  in  Fig.  15.  An  enlarged  view  of  the  dyna- 


FIG.  17.    THE  DYNAMOMETER. 

mometer  is  given  in  Fig.  17.  The  recording  elements  consist 
of  a  Tabor  gas-engine  indicator,  held  by  a  suitable  frame  so 
that  the  pull  compresses  the  spring,  marking  a  line  on  the  drum 

Dee.  1, 1914 
Scale  1  =  103  # 

Av.  Pull 
=  42  Pounds 
High  Gear 
— Declutched 


Beginning 


CHART   IV. 
Dynamometer  Diagram. 


while  the  latter  rotates  under  control  of  a  clock.     A  sample 
diagram  from  the  dynamometer  is  shown  in  Chart  IV. 


PRACTICAL   TESTING    OF    MOTOR-VEHICLES 


55 


TRACTION   DRUMS 

For  power  measurements  the  rear  wheels  are  placed  on  drums 
whose  top  faces  are  level  with  the  floor,  while  the  front  wheels 
remain  at  rest  holding  that  end  of  the  car  in  place.  Connections 
are  made  from  the  rear  axle  to  a  permanent  anchorage  by  chains 


FIG.  1 8.    MOUNTING  OF  ROLLS  AND  BRAKE. 

and  turnbuckles,  affording  adjustment  to  centre  the  wheels  on 
the  drums  and  to  resist  forward  movement  when  power  is  ap- 
plied. The  drums  have  faces  15  inches  wide,  treads  centred 
53  inches  apart,  and  the  actual  circumference  of  the  drums  is 
17.51  feet;  301  revolutions  of  the  drums  are  equal  to  i  mile. 
It  was  originally  planned  to  measure  the  draw-bar  pull  directly 
from  the  axle  connections,  but  this  has  never  been  carried  out 
owing  to  practical  difficulties. 

Power  measurements  are  made  on  a  Prony  brake-pulley, 
36  inches  diameter  by  8  inches  face,  with  a  water-cooled  rim, 
encircled  by  a  rope  brake.  The  brake  is  conveniently  adjusted 
from  the  operating-table  on  the  main  floor  by  a  hand-wheel  and 
shaft  telescoping  over  a  worm-shaft  on  the  brake-arm.  The  pull 


56  HANDBOOK    OF    CARBURETION 

of  the  brake-arm  is  registered  on  platform  scales  beside  the 
operating- table.  The  arrangement  of  levers  gives  123.4  pounds 
pull  on  the  brake-arm  for  100  pounds  on  the  scales.  The  arm 
of  the  brake  is  made  exactly  equal  to  the  radius  of  the  traction 
drums,  so  that  the  brake-load  is  the  same  as  the  draw-bar  pull. 
The  brake  and  traction  drums  are  shown  in  Fig.  22.  The  strap 
was  originally  made  of  steel  band  lined  with  maple  blocks,  as 
shown  in  the  illustration.  This  has  since  been  changed  to  a  rope 
band  of  four  parallel  strands  of  ^-inch  rope  suitably  tied  to- 
gether. The  action  of  the  rope  has  been  smoother,  and  leaves 
little  room  for  improvement. 

DRUM  FRICTION 

The  force  required  to  rotate  the  traction  drums  with  the 
brake-strap  removed  is  a  necessary  quantity.  This  has  been 
determined  approximately  by  placing  a  car  exactly  central  on 
the  drums  and  measuring  the  draw-bar  pull  at  different  speeds 
by  a  spring  balance.  Thus  far  the  friction  force  has  been  taken 
as  35  pounds,  this  being  the  average  for  cars  of  different  weight, 
the  change  due  to  windage  at  various  speeds  having  been  too 
uncertain  to  be  allowed  for. 


DRAW-BAR  PULL 

The  brake-arm,  being  equal  to  the  radius  of  the  traction 
drums,  permits  the  direct  determination  of  the  draw-bar  pull 
from  the  brake-load  when  the  axle  friction  of  the  drums  is 
included.  The  draw-bar  pull  can  be  computed  from  this 
expression: 

Draw-bar  pull  =  1.234  X  load  on  scales  +  35  pounds. 

The  load  on  the  scales  can  be  read  directly,  using  tare  for 
dead  weight  of  the  brake-arm.  The  only  uncertainty  consists 
of  the  allowance  for  friction  and  windage  of  the  drums.  This 
element  is,  however,  a  small  part  of  the  total  draw-bar  pull, 
except  at  very  light  loads,  and  the  figures  given  above  are 
nearly  correct. 


PRACTICAL  TESTING  OF  MOTOR-VEHICLES  57 

MEASUREMENT  OF  SPEED 

A  Hopkins  electric  tachometer  measures  the  speed  of  the 
traction  drums,  with  the  indicating  dial  mounted  on  the  gauge 
board  in  front  of  the  brake-operator.  This  reads  revolutions  per 
minute  of  the  traction  drums  correctly  within  3  per  cent  at  all 
speeds.  Accurate  measurements  of  speed  are  made  by  a  mechani- 
cal revolution  counter,  driven  by  linkage  from  the  traction  drums. 
This  counter  is  located  at  the  operating-table  beside  the  electric 
tachometer,  where  stop-watch  observations  are  made  simul- 
taneously with  the  counter  readings  at  the  beginning  and  end 
of  each  run. 

GASOLINE  MEASUREMENT 

The  fuel  supply  is  contained  in  a  five-gallon  tank  placed  on 
scales  weighing  to  sixteenths  of  an  ounce;  thence  led  by  a 
rubber  tube  to  the  gasoline  inlet  of  the  carbureter.  The  rubber 
tube  is  sufficiently  flexible  to  allow  accurate  weighing  while  it  is 
attached  to  the  can.  An  electric  connection  through  a  mercury 
well  operates  when  the  beam  drops,  giving  a  bell  signal  for  the 
start  and  end  of  each  run.  This  device  has  proved  very  con- 
venient and  accurate.  One-half  pound  of  gasoline  is  regularly 
used  for  light  loads  and  one  pound  for  larger  loads,  giving  runs 
of  from  two  to  six  minutes'  duration. 

A  hand  air-pump  is  attached  to  the  weighing  tank,  giving  the 
necessary  pressure  to  supply  fuel  to  the  carbureter. 

RADIATOR  AND  EXHAUST 

Since  the  car  is  at  rest  and  only  the  motor,  transmission,  and 
rear  wheels  are  in  motion,  the  radiator  is  deprived  of  the  active 
air  circulation  found  on  the  road.  To  prevent  overheating  the 
cylinders  a  supply  of  cooling  water  is  added  to  the  radiator,  with 
the  overflow  of  hot  water  running  to  waste.  The  temperature 
of  the  escaping  water  is  recorded  and  is  usually  kept  at  160°  F. 

PROPOSED  MODIFICATIONS 

Steps  are  now  being  taken  to  increase  the  convenience  of  the 
operator  of  the  measuring  apparatus,  without  changing  the 


58  HANDBOOK   OF   CARBURETION 

methods  used.  It  is  proposed  to  have  both  the  revolution- 
counter  for  the  traction  drums  and  the  time-clock  connected 
electrically  with  the  scale-beam  for  gasoline  weighing.  In  this 
way  both  these  records  will  be  determined  without  personal 
error  of  the  observers.  A  recording  dynamometer  is  also  planned 
to  give  a  record  of  the  load  on  the  scales,  to  show  the  constancy 
of  the  draw-bar  pull.  This  will  be  used  to  supplement,  not  to 
replace,  the  accurate  weighing  system  in  use. 

A  powerful  fan,  driven  at  variable  speeds,  blowing  air  at  the 
radiator,  is  also  planned.  This  may  obviate  the  need  of  water 
overflow  for  cooling  the  radiator  and  may  also  make  possible 
the  observation  of  full  loads  at  higher  speeds. 

METHOD  OF  TESTING 

Rolling  Resistance 

In  calculating  the  rolling  resistance  the  following  steps  are 
taken : 

(a)  The  tires  are  pumped  to  70  pounds  pressure. 

(b)  The  car  is  towed  on  level  floor,  in  high  gear,  declutched, 
to  obtain  the  pull  by  dynamometer. 

(c)  The  projected  area  of  the  car-body  is  measured,  the  width 
across  the  mud-guards  and  the  height  from  the  running-board 
to  the  top  of  the  wind-shield  (half  up),  or  to  the  top  of  closed  cars. 
Allowance  is  made  for  stream-line  bodies  by  reducing  the  area 
slightly. 

If  the  slow  pull  is  denoted  by  r,  the  projected  area  by  a, 
and  the  speed  in  miles  per  hour  by  S,  the  rolling  resistance  is 
calculated  by  the  formula 

R  =  r  +  .003  a  &  (28) 

This  formula  is  assumed  to  give  the  rolling  resistance,  in  pounds, 
of  the  car  at  various  speeds  on  smooth,  level  road,  comparable 
to  the  laboratory  floor. 

DESCRIPTION  OF  RUNS 

After  the  car  is  placed  on  the  drums  and  the  motor  warmed 
up  by  a  preliminary  trial,  the  following  runs  are  made,  during 


PRACTICAL   TESTING   OF   MOTOR-VEHICLES  59 

which  careful  measurements  are  taken  of  the  load  on  the  scales, 
speed,  gasoline  weight,  temperature  of  cooling  water,  and  time 
of  run: 

Run  i.  At  5  miles  per  hour,  or  slowest  speed  possible,  load 
equal  to  rolling  resistance,  level. 

Run  2.  At  10  miles  per  hour,  load  equal  to  rolling  resistance, 
level. 

Run  3.  At  10  miles  per  hour,  load  maximum  at  that  speed. 

Run  4.  At  20  miles  per  hour,  level  road  resistance. 

Run  5.  At  20  miles  per  hour,  maximum  load. 

Run  6.  At  30  miles  per  hour,  level  road  resistance. 

Run  7.  At  30  miles  per  hour,  maximum  load. 

Run  8.  At  40  miles  per  hour,  level  road  resistance. 

Run  9.  At  40  miles  per  hour,  maximum  load. 

Measurements  are  made  during  the  nine  runs  and  recorded 
on  suitable  log  sheets. 

DIAGRAM  OF  RESULTS 

From  these  records  calculations  are  made  in  two  groups, 
one  for  level  road  conditions  and  one  for  maximum  load,  both 
over  the  entire  range  of  speed  covered.  These  calculations  are 
made  for 

Actual  speed,  miles  per  hour  S. 

Maximum  draw-bar  pull  D. 

Level  road  draw-bar  pull  R. 

Gasoline,  reduced  to  miles  per  gallon,  level  road  /. 

Gasoline,  reduced  to  miles  per  gallon,  full  road  F. 

Effective  draw-bar  pull  (D  —  R)  =  P' . 

Horse-power  at  rear  tires,  level  road  (calculated  from  R)  Y. 

Horse-power  at  rear  tires,  full  load  (calculated  from  D)  Z. 

These  results  are  plotted  and  curves  drawn  as  shown  in 
Chart  V. 

The  curves  drawn  through  the  plotted  points  are  in  three 
groups  and  are  subject  to  a  check.  The  draw-bar  curves  for 
level  road  and  full  load  conditions  will  intersect  at  the  maximum 


60 


HANDBOOK   OF    CARBURETION 


speed  of  the  car.     The  horse-power  curves,  Y,  Z,  will  intersect 
at  the  same  speed  as  given  by  the  first-mentioned  curves. 

The  fuel  curves  for  miles  per  gallon  of  gasoline  are  plotted 


60    M.P.H. 


20  30  40  50 

Speed  M.P.H.  -  High  Gear 


CHART  V. 


below,  for  clearness,  using  the  same  abscissre.     These  curves 
likewise  will  intersect  at  the  point  of  maximum  speed. 


PRACTICAL   TESTING   OF   MOTOR-VEHICLES  61 

After  smoothing  out  the  curves  through  the  plotted  points, 
the  exact  values  of  the  draw-bar  pull,  horse-power,  and  miles  per 
gallon  can  be  read  off  at  any  intermediate  speeds  with  greater 
accuracy  than  the  original  plotted  points. 

TABULAR  REPORT  FORM 
'DATA  FROM  TEST  OF 


Model .  .  . 

For 

Made  at Date .... 

By 


DIMENSIONS  OF  CAR 

Wind-resisting  area  (a) sq.  ft.  Wt.  with  driver Ibs. 

Rolling  resistance  declutched Ibs.  Drive 

Gearratio,  ist 2d 3d 4th 

Tires,  size,  front Make 

Tires,  size,  rear Make 

Tires,  tread,  front Inflation Ibs. 

Tires,  tread,  rear Inflation Ibs. 

Ignition Carbureter 

Fuel  sp.gr at F.  Wt.  per  gal Ibs. 


62 


HANDBOOK   OF   CARBURETION 
POWER  AND  FUEL  MEASUREMENTS 


5 

Speed 
of  Car 
6oN 
3oiT 
Miles 
Hour 

DRAW-BAR  PULL 

FUEL  DATA 

A 

Accel- 
eration 

•322P 

Feet  per 
Sec.  per 
Sec. 

HORSE-POWER 
AT  REAR  TIRES 

R 

D 

P 

/ 

F 

Y 

Z 

On  Level 
Road 
r  +  .003 
aS* 
Pounds 

Max.  at 
Full 
Load 
1.234!, 

Pounds 

Net 
Effective 
Pull 
(D-R)  100 

On 

Level 
Road 
wST 
6op 

Miles 
per 
Gallon 

At 
Full 
Load 
•wST 
6op 
Miles 
<£!. 

Level 
Road 
RS 
375 
H.  P. 

Full 
Load 
DS 
375 
H.  P. 

W 
Pounds 

5 

IO 

2O 

30 

40 

50 

EXPLANATION 

N   =  revolutions  of  drums  during  run  from  records. 
T    =  time  of  run,  minutes,  from  records. 
r     =  rolling  resistance,  slow  speed,  by  dynamometer,  Ibs. 
L    =  load  on  brake-scale  beam  during  run,  Ibs. 
W  =  weight  of  car,  including  driver,  Ibs. 
w    =  weight  of  one  gallon  of  gasoline,  Ibs. 
p    =  Ibs.  of  gasoline  used  during  run  from  records. 
a     =  wind-resisting  area  of  car-body,  measured. 
S    =  speed  of  car  in  miles  per  hour. 

EXPRESSION  OF  RESULTS 

As  noted  in  the  introduction  to  this  paper,  a  test  of  per- 
formance should  be  of  value  to  the  commercial  as  well  as  to  the 
technical  end  of  the  automobile  industry.  In  order  to  be  in- 
telligible to  other  than  the  trained  engineer,  results  should  be 
expressed  in  non-technical  terms  of  common  usage.  At  the  same 
time  the  expression  of  results  must  omit  no  detail  of  desirable 
information. 

On  close  inspection  these  conditions  become  less  difficult 
than  they  at  first  appear.  The  satisfaction  of  a  motor-car 
owner  is  dependent  upon  four  factors,  so  far  as  performance  is 
concerned: 


PRACTICAL   TESTING   OF  MOTOR-VEHICLES 


63 


First,  the  car  must  have  a  wide  speed-range  on  the  high  gear ; 
second,  it  must  accelerate  quickly;    third,  it  must  possess  sum- 


Pounds  Effective  Pull  per  100  Pounds  Car  Wt.  =  P 
Negotiable  Grade  -  Per  cent 

_o  re  *.  a>  GO  o  J«  S 

40 

^  ? 
»    * 
t-»u. 

**7 

o 

1 

10  § 

^  

e  % 

N 

\ 

\ 

^ 

•  

1 

—-^ 

x 

v  —  •  — 

•^  — 

F 

—  .  i 

,• 

T^ 

\ 

\ 

10                20                30                40                50                60                 70 

Speed  M.P.H.  -  High  Gear 
CHART  VI. 

Car  No.  I.  1913  Roadster.  6  cylinders.  Bore,  4  inches.  Stroke,  5^ 
inches.  Weight,  with  driver,  4,435  pounds.  Rolling  resistance,  80  pounds. 
Wind-resisting  area,  21.8  square  feet.  Tires,  37x5,  non-skid.  Inflation, 
70  pounds  rear,  60  pounds  front. 

cient  power  to  negotiate  grades,  or  to  overcome  heavy  road 
conditions;  fourth,  it  must  be  economical  of  fuel. 

ACCELERATION  AND  HILL-CLIMBING  ABILITY 

The  second  and  third  factors  are  direct  functions  of  the 
excess  power  of  the  car.  By  "excess  power"  is  meant  the  total 
effort  of  the  engine  minus  the  total  rolling  resistance.  In  other 
words,  it  is  the  excess  of  pull  of  which  the  car  is  capable  at  any 


64  HANDBOOK   OF   CARBURETION 

speed,  exerted  on  the  roadway,  over  and  above  the  pull  necessary 
to  move  the  car  against  its  own  rolling  resistance  at  that  speed. 

In  brief,  it  is  the  net  effective  power  of  the  car  and  may  be 
conveniently  expressed  in  pounds  pull  and  designated  by  P'  . 
Concretely,  P'  is  determined,  as  already  noted,  by  subtracting 
the  total  rolling  resistance  from  the  maximum  draw-bar  pull, 
as  determined  by  the  methods  herein  outlined. 

If  the  net  effective  oower  P'  be  considered  as  force,  we  have 

P'  =  MA 
where 

W 

M  =  Mass  or  — 
g 
then  if 

A  =  acceleration  in  feet  .per  sec.  per  sec. 
W  =  weight  of  car  (with  driver)  in  pounds. 
G  =  per  cent  grade  that  can  be  surmounted. 


W 

IPO  P 


(29) 


If,  therefore,  we  reduce  P'  to  P,  which  equals  the  net  effective 
power  per  100  pounds  of  car  weight,  the  same  scale  gives  a  direct 
reading  of  the  maximum  gradient  a  car  will  surmount  at  a  given 
speed,  while 

A  =  0.322  P  (30) 

SPEED  RANGE 

Plotting  the  curve  of  net  effective  power  per  100  pounds  of 
car  weight  with  pounds  for  a  unit,  as  ordinates  and  speed,  in 
miles  per  hour,  as  abscissae,  the  point  where  the  P  curve  falls  on 
the  zero  line  of  power  establishes  the  maximum  speed  of  the  car. 
The  minimum  speed  at  full  load  is  designated  by  the  opposite 
end  of  the  P  curve,  while  the  minimum  speed  on  level  road  is 
shown  by  the  left-hand  of  the  /  curve,  both  as  established  by 
observation  during  the  test. 


PRACTICAL   TESTING   OF   MOTOR-VEHICLES 

FUEL  CONSUMPTION 


65 


From  the  foregoing  it  is  seen  that  a  single  curve  expresses 
three  of  the  four  desired  factors.    There  remains  only  fuel  con- 

14r- 


12 


I     10 
fl 

i! 

!l 
§1 

Si 

t5g 

S» 

P 

i 


10  20  30  40  50  60  70 

Speed  M.P.H.- High  Gear 

CHART  VII. 

Car  No.  2.  1915  Touring  Car.  6  cylinders.  Bore,  4  inches.  Stroke,  5^2 
inches.  Weight,  with  driver,  4,950  pounds.  Rolling  resistance,  83  pounds. 
Wind-resisting  area,  22.6  square  feet.  Tires,  37  x  5,  plain  tread.  Inflation, 
70  pounds  front  and  rear.  Gear  ratio,  direct  drive,  3.53. 

sumption.     As  this  item  covers  a  range  from  the  lightest  to  the 
heaviest  loads,  it  seems  best  to  plot  both  extremes. 

This  is  conveniently  done  on  the  same  chart  by  renumbering 
the  ordinates  on  the  right  of  the  diagram,  using  as  a  standard 
the  common  unit  of  miles  per  gallon.  Minimum  fuel  con- 
sumption then  becomes  maximum  mileage  and  is,  of  course,  the 
mileage  possible  on  level  cement  road.  This  may  be  designated 
by  /.  The  mileage  at  full  load  may  be  designated  by  F. 


66 


HANDBOOK   OF   CARBURETION 


FUEL  CHECK  ON  SPEED  LIMIT 

As  already  noted,  it  is  evident  that  maximum  speed  on  level 
road  is  identical  with  full  load  at  the  same  speed.     It  is  therefore 


\ 


\ 


0  10  20  30  40  50  60  70 

Speed  M.P.H.  -  High  Gear 

CHART  VIII. 

Car  No.  3.  1915  Touring  Car.  6  cylinders.  Bore,  4  inches.  Stroke, 
5X  inches.  Weight,  with  driver,  4,562  pounds.  Rolling  resistance,  40  pounds. 
Wind-resisting  area,  19.2  square  feet.  Tires,  36x4^,  cord.  Inflation, 
70  pounds  front  and  rear.  Gear  ratio,  direct  drive,  3.78. 

clear  that  the  /  and  F  curves  should  join  on  the  same  abscissas 
where  the  P  curve  reaches  a  zero  value.  This  affords  a  positive 
check  on  the  accuracy  of  the  observations  and  plotting.  Charts 
VI  to  XI  show  the  efficacy  of  this  check  and  its  corrective 
influence  on  the  characteristics  of  all  curves. 


PRACTICAL  TESTING   OF  MOTOR-VEHICLES 


67 


The  influence  of  the  termination  of  the  curves  is  clearly  shown 
in  Chart  VIII.  The  last  observation  on  the  /  curve  indicates  a 
maximum  speed  somewhat  higher  than  is  shown  in  the  diagram, 


0  10  20  30  40  50  60  70 

Speed  M.P.H.  -  High  Gear 

CHART   IX. 

Car  No.  4.  1915  Touring  Car.  6  cylinders.  Bore,  3fg  inches.  Stroke, 
4  inches.  Weight,  with  driver,  3,020  pounds.  Rolling  resistance,  45  pounds. 
Wind-resisting  area,  20.2  square  feet.  Tires,  34  x  4X1  cord.  Inflation,  70 
pounds  front  and  rear.  Gear  ratio,  direct  drive,  3.71. 

whereas  the  P  curve  plots  smoothly  to  the  speed  limit  shown. 
This  discrepancy  was  doubtless  due  to  manual  adjustment  of 
the  carbureter  by  the  dash  control. 

Chart  IX  shows  two  actual  observations  of  fuel,  falling 
practically  together  on  a  point  in  close  agreement  with  the  speed 
limit  as  determined  by  the  P  curve.  •- 


HANDBOOK   OF   CARBURETION 


SPEED  LIMIT  OF  OBSERVATIONS 

In  the  diagrams  shown  herewith,  but  one,  Chart  X,  is  incom- 
plete through  lack  of  additional  observations  at  higher  speeds. 
On  account  of  instances  like  this  it  is  desirable  to  determine 
points  on  the  curves  at  as  high  vehicle  speeds  as  possible.  Usu- 
ally it  is  inexpedient  to  run  the  car,  particularly  at  full  load,  at 
speeds  exceeding  40  miles  per  hour  because  as  the  car  is  station- 
ary there  is  a  tendency  to  overheating  through  the  absence  of 
the  cooling  effect  of  the  motion  of  the  car  on  the  road.  Ordinarily, 
however,  a  sufficient  number  of  observations  may  be  taken  at 
and  below  40  miles  per  hour  to  establish  reasonable  accurate 
projection  of  the  curves. 

APPLICABILITY  TO  ROAD  CONDITIONS 

The  true  value  of  this  method  of  testing  depends  largely 
upon  the  fidelity  with  which  its  results  can  be  duplicated  on 
the  road.  To  establish  this,  several  road  checks  have  been 
conducted  by  the  authors  under  strictly  test  conditions,  and  by 
others  under  ordinary  conditions  of  driving.  For  example,  a 
four-cylinder  car  was  driven  over  a  practically  level  course  of 
2,801  feet  on  Orange  Street,  New  Haven,  a  road  surface  corre- 
sponding closely  to  that  of  the  laboratory  floor.  The  throttle 
was  set  in  various  marked  positions  and  the  speed  accurately 
noted  by  a  stop-watch.  Tests  were '  duplicated  with  the  car 
driven  in  both  directions  to  eliminate  the  effect  of  any  possible 
slight  grade.  The  car  was  then  placed  on  the  test  stand  and  the 
throttle  opened  to  the  same  positions.  Following  is  a  tabulation 
of  the  results: 

COMPARISON  OF  TEST-STAND  AND  ROAD-TEST  RESULTS 


SPEED 

DRAW-BAR  PULL 

On  Road 
M.  P.  H. 

On  Test-Stand, 
M.  P.  H. 

By  Formula, 
Pounds 

Actual, 
Pounds 

Error, 
Per  Cent 

I4-83 

14.27 

57-6 

58.0 

0.7 

18.84 

18.23 

65.2 

66.5 

2.0 

23.00 

22.23 

75-1 

76.7 

2.12 

26.90 

27-75 

86.2 

88.0 

2.09 

30.40 

30.95 

97.6 

102.8 

5-33 

PRACTICAL   TESTING   OF   MOTOR-VEHICLES  69 

Car  No.  i  of  the  present  series  of  tests  was  driven  go  miles 
by  the  owner  on  selected  macadam  roads  and  fell  but  0.6  of  a 
mile  per  gallon  below  the  average  shown  by  the  block  test 
between  the  same  speed  limits. 


D. 

II 

£ 

w^— 

^ 

X 

\p 

Pounds  Efifective  Pull  per  100  Pounds  Ci 
Negotiable  Grade  -  Per  cent 

ft  *.  0»  CO  .j 

x 

\ 

^ 

30 

II 
.& 

S  ! 
Mi.per  Gal. 

/ 

U—  -""""""" 

0 

0 

.--- 

^ 

-^ 

F 

10                20                30                40                50                60                70 

Speed  M.P.H.  -  High  Gear 
CHART  X. 

Car  No.  5.  1915  Touring-Car.  8-cylinder.  Bore,  3^  inches.  Stroke, 
51/6  inches.  Weight,  with  driver,  4,020  pounds.  Rolling  resistance,  78.5 
pounds.  Wind-resisting  area,  18.6  square  feet.  Tires,  36x4^2,  non-skid. 
Inflation,  70  pounds  front  and  rear.  Gear  ratio,  direct  drive,  5.02. 

Another  car,  showing  11.2  miles  per  gallon  under  test,  was 
actually  driven  n.i  miles  over  a  selected  road  with  a  carefully 
weighed  gallon  of  gasoline.  This  duplication  of  test-stand 
results  by  different  drivers  on  different  roads  must  be  con- 
sidered as  something  more  than  coincidence,  and  its  testimony 
lends  weight  to  the  accuracy  of  the  method. 


70 


HANDBOOK  OF  CARBURETION 
DISCLOSURE  OF  CHARACTERISTICS 


This  method  also  shows  to  a  surprising  degree  the  relative 
action  of  certain  parts  of  different  cars.  If  the  car  is  equipped 
with  manual  spark  control  and  dash  control  of  the  carbureter, 
it  also  shows  the  relative  skill  of  different  drivers  and  the  effect 


11 
I*. 


\ 


\ 


Speed  M.P.H.  -  High  Gear 

CHART  XL 

Car  No.  6.     1915  Touring-Car.     6-cylinder.     Bore,  4^2  inches.     Stroke, 
5#    inches.     Weight,    with   driver,    5,020    pounds.     Rolling    resistance,    47 
pounds.     Wind-resisting  area,  22.3  square  feet.     Tires,   37  x  5,   cord. 
flation,  70  pounds,  front  and  rear.     Gear  ratio,  direct  drive,  3.5. 


In- 


of these  adjustments  in  the  hands  of  the  average  user  may  be 
learned  therefrom. 

The  test  shows  the  performance  of  the  car  as  it  is  at  the 
moment  of  testing.  What  difference  another  make  of  tires,  a 
different  adjustment  of  the  carbureter,  or  the  change  of  any 


PRACTICAL   TESTING   OF   MOTOR-VEHICLES 


71 


other  feature  would  make  can  be  determined  only  by  repetition 
of  the  test  under  the  new  conditions.  For  instance,  in  Chart  IX, 
the  driver  evidently  desired  his  car  to  establish  a  reputation  for 


Q. 

i- 

I! 

Si 
ll 


II 


\ 


\ 


10 


30  40 

Speed  M.P.H.  -  High  Gear 


50 


70 


CHART  XII. 

Renault  Touring-Car.     Plotted  from  test  by  Dr.  A.  Riedler  (see  Chart 
XIII). 

fuel  mileage.  He  was  successful,  but  at  the  expense  of  speed, 
acceleration,  and  hill-climbing  ability. 

Again,  certain  characteristics  of  the  carbureter  are  clearly 
shown.  For  instance,  the  performance  of  the  carbureter  in 
Car  No.  i  (Chart  VI)  was  wholly  consistent,  giving  smooth 
curves  at  all  speeds,  a  fuel  consumption  at  full  load  directly 
proportional  to  the  speed,  and  a  maximum  mileage  on  level  road 
at  between  20  and  25  miles  per  hour. 

Car  No.  3  (Chart  VIII)  shows  equally  good  action  at  full 


72 


HANDBOOK   OF   CARBURETION 


1 


M  M 


PRACTICAL     TESTING     OF     MOTOR-VEHICLES  73 


0  10  20  30  40  50  60  70 


CHART  XIV. 

Comparison  of  fuel  consumption  and  power  of  six  representative  American 
cars.     Cars  Nos.  I  to  6,  inclusive. 


74  HANDBOOK   OF   CARBURETION 

load,  but  faulty  compensation  under  throttle,  with  resulting 
maximum  mileage  at  minimum  speed.  That  this  is  really  a 
characteristic  of  the  carbureter  is  shown  by  comparing  the  same 
make  of  carbureter  on  a  different  car,  Car  No.  6  (Chart  XI), 
which  exhibits  the  same  pronounced  characteristics. 

Note  also  the  similarity  of  the  character  of  both  fuel  curves 
in  Car  No.  2  (Chart  VII),  and  compare  them  with  similar 
characteristics  developed  by  a  different  carbureter  on  Car 
No.  4  (Chart  IX). 

Again,  the  rolling  resistance  of  cars  of  approximately  the 
same  weight  is  found  to  vary  markedly.  Whether  this  is  due  to 
different  tires  or  to  internal  friction  can  only  be  determined  by 
substituting  in  one  case,  or  by  more  detailed  investigation  in 
the  other. 


THE  PERFORMANCE  TEST  AS  A  BASIS  FOR  SUBSEQUENT 
INVESTIGATION 

The  possibilities  of  this  method  for  maintaining  constant 
all  conditions,  except  the  one  under  investigation,  offer  alluring 
opportunity  for  the  investigation  of  various  components  entering 
motor-car  construction.  The  development  of  the  method  of 
testing  herein  outlined  has,  of  itself,  occupied  so  much  of  their 
time,  that  the  enticing  field  of  detailed  analysis  of  the  results 
has  hardly  been  entered  by  the  authors. 

Suggestions  concerning  more  detailed  investigation  are  out- 
side the  province  of  this  paper,  but,  in  illustration  of  the  possi- 
bilities, Chart  XII  is  an  expression,  by  the  proposed  method, 
of  a  test  of  a  Renault  touring-car  made  by  Dr.  A.  Riedler,  of 
Berlin,  Germany.  Chart  XIII  is  a  reproduction  of  Dr.  Riedler's 
complete  test  as  published  in  a  translation  of  his  work  entitled 
"The  Scientific  Determination  of  the  Merits  of  Automobiles." 
In  this  diagram  Dr.  Riedler  has  shown  the  sources  of  loss  from 
different  causes,  expressed,  as  the  authors  believe,  unfortunately 
in  terms  of  horse-power.  Chart  XII  may  be  said  to  indicate 
an  effect,  while  Chart  XIII  is  an  analysis  of  the  cause.  The 
facts  of  Chart  XII  may  be  accurately  determined  in  two  hours, 


PRACTICAL  TESTING  OF  MOTOR-VEHICLES  75 

while  those  of  Chart  XIII  can  be  learned  only  after  the  most 
protracted  and  painstaking  effort. 

DRAW-BAR  PULL  vs.  HORSE-POWER 

The  authors  can  not  close  without  recording  a  protest  against 
the  use  of  horse-power  as  a  unit  for  motor-car  rating.  Whatever 
may  be  its  value  in  the  classification  of  motor-car  engines,  it 
seems  utterly  inconsistent  to  apply  it  to  the  performance  of  a 
vehicle.  It  is  the  pull  or  push  of  the  tire  on  the  road  that  is 
effective  in  the  propulsion  of  a  car.  Witness  the  utter  absurdity 
of  a  steam-car  equipped  with  a  20  horse-power  engine,  out- 
pacing and  outclimbing  gas-cars,  the  engines  of  which  will 
develop  upward  of  80  horse-power  on  the  block.  The  steam-car 
accomplishes  this  by  greater  and  more  uniform  torque  (or 
turning-moment)  delivered  to  its  rear  wheels  through  the  con- 
tinued and  overlapping  admission  of  high  cylinder  pressures; 
therefore,  it  is  clearly  this  torque,  or  turning  effort,  that  should 
be  recognized,  and  its  direct  and  easily  measurable  result,  draw- 
bar pull,  seems  to  be  the  logical,  final  unit  of  such  measurement. 


CHAPTER  V 

DIRECT  DETERMINATION   OF  CARBURETER  ACTION 

As  HAS  been  shown  in  preceding  chapters,  the  primary 
function  of  a  carbureter  is  to  maintain  the  relative  proportions 
of  gas  and  air  in  an  explosive  mixture.  The  direct  determina- 
tion of  how  well  this  function  is  performed  is  attended  with 
difficulties. 

The  amount  of  fuel  entering  the  mixture  may  be  accurately 
measured  by  ordinary  means.  The  air  content  is  by  no  means  so 
easy  of  determination.  The  problem  is  complicated  by  the  fact 
that  in  the  internal  combustion  engine  the  air-flow  is  induced  by 
a  series  of  more  or  less  separate  impulses,  so  that  the  pressure 
flow  is  pulsating  in  character.  The  result  of  this  is  to  introduce 
inertia  effects  and  other  influences,  which  react  on  the  velocity 
of  the  flow  to  such  an  extent  as  to  make  its  accurate  determina- 
tion exceedingly  difficult. 

THE  ANEMOMETER 

The  anemometer,  or  other  form  of  mechanical  meter,  is  not 
sufficiently  responsive  to  the  frequent  pulsations  even  if  the 
errors  inherent  in  such  instruments  could  be  tolerated. 


ORIFICE  IN  THIN  PLATE 

Attempts  have  been  made  to  measure  the  flow  through  known 
orifices  in  thin  plate  into  a  chamber  in  which  pressures  are 
indicated  by  means  of  a  manometer.  The  chief  difficulty 
experienced  with  this  apparatus  is  the  determination  and  main- 
tenance of  the  actual  coefficient  of  flow.  This  varies  with  the  size 
of  the  orifice  and  with  the  pressure,  density,  and  velocity  of 
the  air — all  variable  conditions. 

76 


DIRECT  DETERMINATION   OF   CARBURETER  ACTION  77 

Formula  of  Flow 

Durley, Trans.  A.S.M.E.,  Vol.  XXVII,  page  193,  shows  that  if 
w  =  weight  of  gas  discharged  per  second  in  Ibs. 
Pi  =  pressure  inside  orifice  in  Ibs./sq.  foot. 
P2  =  pressure  outside  orifice  in  Ibs./sq.  foot. 
7  =  ratio  of  specific  heat  at  constant  volume  to  that  at 

constant  pressure. 
d  =  diameter  of  orifice  in  inches, 
then  for  air  at  60°  F.  when  7  =  1.404 


0.000491  d2Pi\  (r)  1.425  -  \j    1-712  (31) 


If  we  neglect  the  changes  of  density  and  temperature  occurring 
as  the  air  passes  through  the  orifice  we  obtain  a  simpler,  though 
approximate  formula  for  the  ideal  discharge. 


w  =  0.01369  d*\  -^  (32) 

in  which 

d  =  diameter  of  orifice  in  inches. 
i  =  difference  in  pressure  measured  in  inches  of  water. 
P  =  mean  absolute  pressure  in  Ibs./sq.  ft. 
T  =  absolute  temperature  in  F.°  =  F.°  -f  461. 

Thickness  of  Plate 

Up  to  pressures  of  about  20  inches  of  water,  the  results  of  the 
foregoing  formulae  agree  very  closely.  At  higher  differences  of 
pressure  divergence  becomes  noticeable.  The  values  found  by 
these  formulae  are  to  be  multiplied  by  a  coefficient  c,  determined 
experimentally.  They  hold  good  only  for  orifices  of  the  particu- 
lar form  experimented  with  and  bored  in  plates  of  the  same 
thickness,  viz.:  iron  plates  0.057  inches  thick. 

Necessary  Conditions 

Experiments  and  curves  plotted  from  them  indicate  that  up 
to  a  pressure  of  about  20  inches  of  water 


78 


HANDBOOK   OF    CARBURETION 


(ia)  The  coefficient  for  small  orifices  increases  as  the  head 

increases. 

(ib)  For  a  2-inch  orifice,  the  coefficient  is  almost  constant, 
(ic)  For  orifices  larger  than  2  inches,  the  coefficient  decreases 

as  the  head  increases  and  at  a  greater  rate  the  larger 

the  orifice. 

(2)  The  coefficient  decreases  as  the  diameter  of  the  orifice 

increases  and  at  a  greater  rate  the  higher  the  head. 

(3)  The  coefficient  does  not  change  appreciably  with  tempera- 

ture between  40°  and  100°  F. 

(4)  The  coefficient  (at  heads  under  6  inches)  is  not  appreciably 

affected  by  the  size  of  the  box  in  which  the  orifice 
is  placed,  if  the  ratio  of  the  areas  of  the  box  and  orifice 
is  at  least  20:  i. 


TABLE  I 

COEFFICIENT  OF  DISCHARGE  (c)  FOR  VARIOUS  HEADS  IN  INCHES  OF  WATER 
AND  DIAMETERS  OF  ORIFICE  IN  INCHES,  IN  PLATE  0.057  INS.  THICK. 


Diameter 
of 
Orifice 

i-Inch 
Head 

2-Inch 
Head 

3-Inch 
Head 

4-Inch 
Head 

5-Inch 
Head 

V.6 

0.603 

0.606 

0.610 

0-613 

0.616 

X 

0.602 

0.605 

0.608 

0.610 

0-613 

I 

0.601 

0.603 

0.605 

0.606 

0.607 

iH 

0.601 

0.601 

0.602 

0.603 

0.603 

2 

0.600 

O.6OO 

0.600 

0.600 

0.600 

*y> 

0-599 

0-599 

0-599 

0.598 

0.598 

3 

0-599 

0.598 

0-597 

0.596 

0.596 

3# 

0-599 

0-597 

0.596 

0-595 

0-594 

4 

0.598 

0-597 

0-595 

0-594 

0-593 

4^ 

0.598 

0.596 

0-594 

0-593 

0.592 

APPARATUS  FOR  CARBURETER  MEASUREMENTS 

For  purposes  of  carbureter  measurements  orifices  of  various 
diameters  may  be  bored  in  a  plate  0.057  inches  thick,  forming 
one  side  of  a  closed  box.  Provision  should  be  made  to  close  all 
orifices  but  the  one  in  use.  In  accordance  with  provision  4  of 
the  preceding  paragraph,  the  cross-sectional  area  of  this  box 
should  be  at  least  twenty  times  the  area  of  the  largest  orifice. 


DIRECT  DETERMINATION   OF   CARBURETER   ACTION  79 

Rubber  Diaphragm 

One  side  of  the  box  is  made  of  sheet  rubber,  the  flexibility  of 
which  aids  materially  in  neutralizing  the  pulsations  of  the  air- 
current.  The  box  is  provided  with  a  thermometer  and  is  con- 
nected to  one  leg  of  a  manometer  graduated  in  inches  and  tenths. 
Connection  is  made  from  the  box  to  the  carbureter  by  means  of 
suitable  piping.  If  the  carbureter  has  more  than  one  intake 
opening  it  is  well  to  inclose  the  entire  instrument  in  an  air-tight 
box  and  connect  this  box  to  the  meter-box  by  a  pipe. 

The  carbureter-box  may  be  made  of  sheet  metal  with  one 
side  acting  as  a  cover,  secured  in  place  against  an  air-tight 
gasket.  The  box  is  supported  between  the  carbureter  and  mani- 
fold flanges,  being  bored  to  register  with  manifold  passage  and 
with  the  cap  screws  securing  the  flanges.  Tightness  is  secured  by 
gaskets  between  both  flanges  and  the  box. 

It  seems  unnecessary  to  add  that  every  precaution  must  be 
taken  to  guard  against  air  leakage  with  the  apparatus,  and  to 
this  end  all  joints  must  be  made  air-tight. 

With  an  apparatus  so  constructed,  the  weight  of  air  used  in 
the  carbureter  may  be  determined  by  a  derivation  from  Durley's 
formulae,  substituting  observed  values  in  the  following  equation: 


\m  (70.748,6  -  5.184^) 
w  =  0.01369  c  d*\  -  —^r—  (33) 

when 

c  =  a  coefficient  selected  from  Table  I. 
w  =  weight  of  air  used  in  Ibs./sec. 
d  =  diameter  of  orifice  in  inches. 
m  =  manometer  reading  in  inches  of  water. 
B  =  barometric  pressure  of  the  atmosphere  in  inches  of 

mercury. 
T  =  absolute  temperature  of  the  air  =  F.°  +  461. 

Orifice  Diameters 

The  diameter  of  the  orifice  should  be  selected  with  a  view  to 
maintaining  a  manometer  reading  sufficiently  high  at  the  lowest 


80  HANDBOOK   OF   CARBURETION 

engine  speeds  to  insure  accuracy  of  observation.  As  the  speed 
increases,  larger  diameters  should  be  employed  so  the  head  of 
water  is  kept  as  low  as  is  consistent  with  convenient  manipulation. 

Objection 

One  of  the  principal  objections  to  this  method  is  that  the 
carbureter  is  at  all  times  operating  at  sub-atmospheric  pressures, 
a  condition  which  may  not  be  fairly  comparable  to  its  operation 
in  actual  service.  This  error  may  be  materially  reduced,  how- 
ever, by  employing  orifices  which  give  low  readings  on  the 
manometer,  substituting  larger  orifices  as  the  demand  for  air 
increases. 

This  manipulation  furthermore  reduces  the  error  in  the 
coefficient  of  flow,  which  may  be  easily  kept  within  i  per  cent. 

THE  VENTURI  METER 
Principles  Involved 

Probably  the  most  practical  method  for  the  direct  measure- 
ment of  air  is  by  the  use  of  the  Venturi  meter.  This  instrument 
depends  for  its  action  on  the  loss  of  head  caused  by  the  increased 
velocity  of  flow  through  a  constriction  in  the  cross-sectional  area 
of  a  tube.  By  equation  (4)  (Chapter  I),  this  loss  of  head,  h,  is 

Va* 

ft    — 

2g 

therefore,  by  a  measurement  of  the  loss  of  head,  h,  we  are  enabled 
to  determine  the  velocity  by  the  formula 

Va  =  c 

when  c  =  a  coefficient  of  flow. 

When  the  construction  of  the  tube  is  made  with  highly  finished 
surf  aces,  at  angles  which  closely  follow  the  natural  contraction 
of  the  vein  of  flow,  this  coefficient  is  nearly  constant  at  greater 
than  0.98. 

The  head  or  pressure  difference  is  measured  in  inches  of  water 


DIRECT   DETERMINATION   OF   CARBURETER  ACTION  81 

by  connecting  one  leg  of  a  manometer  U  tube  with  the  "up- 
stream" end  of  the  meter,  while  the  other  is  connected  to  the 
throat. 

Calibration 

The  manufacturers  of  these  meters  furnish  a  calibration 
curve  of  each  instrument,  showing  the  actual  discharge  in  cubic 
feet  per  minute  under  standard  conditions  of  temperature  and 
barometer,  viz.,  62°  F.  and  29.92  inches  of  mercury. 

Barometric  and  Temperature  Correction 

For  any  other  temperature  and  pressure,  the  discharge  may 
be  determined  by  the  formula 


7  =  0.24    Af  (34) 

when 

V  =  volume  of  air  in  cubic  feet  per  minute. 
M  =  meter  reading  in  cubic  feet  per  minute. 

T  =  absolute  temperature  of  the  atmosphere  =  (F.°  +  459). 

B  =  barometer  reading  in  inches  of  mercury. 
As  air/gas  ratios  are  usually  given  by  weight,  the  weight  of  air 
in  pounds  per  minute  (W)  is  found  by  the  following: 


W  =  0.31835  M^  (35) 

Application  to  Carbureter  Measurements 
As  is  the  case  with  the  orifice  in  thin  plate,  it  is  desirable  to 
neutralize  pulsations  in  the  air-current  by  a  flexible  diaphragm 
forming  one  side  of  a  box  into  which  the  air  is  metered  and  from 
which  it  is  withdrawn  to  the  carbureter. 

The  use  of  at  least  two  sizes  of  meters  is  also  desirable:  one 
with  a  throat  diameter  about  0.5  inches,  and  the  other  with  a 
throat  diameter  of  from  2  to  3  inches.  The  former  indicates  from  4 
to  26  cubic  feet  per  minute,  with  from  0.5  to  18  inches  difference 
in  the  head  of  water  in  the  U  tube,  while  the  latter  discharges 


82  HANDBOOK  OF   CARBURETION 

25  to  200  cubic  feet  per  minute,  with  manometer  deflections  of 
from  0.2  to  1 6  inches.  By  substituting  one  instrument  for  the 
other,  accurate  measurement  may  be  secured  over  a  range 
sufficient  to  cover  small  engines  at  slowest  speeds  or  large  engines 
at  highest  speeds. 


CHAPTER  VI 
CHEMISTRY  OF  CARBURETION 

Complexities 

THE  performance  of  an  automobile  engine  presents  problems 
of  a  physicochemical  nature.  Because  of  the  complexities  of  the 
interrelationship  of  these  two  branches  of  science,  the  chemical 
investigation  of  combustion  reactions  and  their  physical  effect 
on  power  output  has  registered  less  progress  than  its  importance 
deserves.  This  seems  due  in  large  measure  to  a  lack  of  under- 
standing cooperation  between  chemist  and  physicist,  and  it  is 
therefore  gratifying  to  note  the  increasing  interest  shown  by 
engineers  in  the  study  of  exhaust  gases. 

Primarily,  power  is  developed  in  the  internal  combustion 
engine  as  the  direct  effect  of  heat  liberated  solely  by  means  of 
certain  chemical  reactions  known  as  combustion.  Knowledge 
of  these  reactions  is,  therefore,  of  prime  importance.  Because 
combustion  takes  place,  after  a  fashion,  throughout  such  a  wide 
range  of  mixture  composition,  engineers  are  prone  to  lose  sight 
of  the  necessity  for  a  careful  study  of  these  fundamental  reactions, 
even  though  they  form  the  very  basis  of  power  development. 

Availability  of  Exhaust  Gas  Analysis 

Analysis  of  the  exhaust  gases  from  an  internal  combustion 
engine  furnishes  one  of  the  most  convenient  methods  of  com- 
paring carbureter  performances.  It  must  be  admitted  that  gas 
analysis  has  not  yet  reached  the  point  where  complete  infor- 
mation may  be  obtained,  but  with  the  well-known  methods 
in  common  use  sufficient  data  may  be  obtained  which,  when 
properly  interpreted,  will  be  found  wholly  consistent  and  in 
point  of  fact  sufficiently  accurate  for  all  practical  purposes. 

Before  the  true  value  of  gas  analysis  can  be  fully  realized, 
83 


84  HANDBOOK   OF   CARBURETION 

an  intimate  knowledge  of  the  chemical  and  thermal  reactions 
taking  place  within  the  cylinder  is  necessary. 

COMBUSTION 

The  fuel  used  in  automobile  engines  is  a  hydrocarbon,  or 
really  a  combination  of  several  hydrocarbons  forming  part  of 
what  is  known  as  the  paraffin  series.  These  liquid  distillates, 
obtained  from  crude  petroleum,  have  the  general  chemical 
formula  CWH2W  +  2,  which  may  be  explained  as  a  substance  com- 
posed of  n  molecules  of  carbon  and  211  +  2  molecules  of  hy- 
drogen. Gasoline,  for  example,  is  composed  largely  of  hexane, 
which  contains  6  molecules  of  carbon,  namely,  n  =  6  combined 
with  2X6  +  2  =  14  molecules  of  hydrogen  =  C6Hi4. 

Reactions  of  .Hexane 

The  term  combustion  may  be  denned  as.  the  union  of  a  sub- 
stance with  oxygen.  Both  hydrogen  and  carbon,  when  raised 
to  the  required  temperature,  in  the  presence  of  air  unite  very 
readily  with  the  oxygen  in  the  air,  the  hydrogen  forming 
water  and  the  carbon,  carbon-dioxide,  CO2,  when  the  proper 
quantity  of  air  is  present,  or  carbon  monoxide,  CO,  when  there 
is  insufficient  air  for  complete  combustion.  These  reactions 
may  be  expressed  as  follows,  if  we  assume  gasoline  to  be  com- 
posed entirely  of  hexane  : 

2C6H14  +  i9O2-f  7i.3N2  =  i2CO2+i4H2O  +  7i.3N2  (36) 
This  equation  shows  perfect  combustion  in  which  all  the  carbon 
is  oxidized  to  CO2  and  all  the  hydrogen  has  formed  water. 

The  following  equation  presupposes  an  insufficient  amount  of 
air  and  its  consequent  imperfect  combustion: 


=  9C02  +  3CO  +  14  H20  +  65.8N2  (37) 
Here  the  oxidation  of  the  carbon  has  been  incomplete,  resulting 
in  the  formation  of  both  CO2  and  CO. 

Relative  Volume  of  the  Exhaust 

From  equation  (36)  we  find  that  2  volumes  of  hexane  unite 
with  90.3  volumes  of  air  and  that  the  exhaust  gas  occupies 


CHEMISTRY   OF    CARBURETION  85 

12  -f-  14  +  71.3  =  97.3  volumes.  As  the  14  volumes  of  H2O, 
existing  in  the  exhaust  as  steam,  promptly  condense,  the  final 
exhaust  consists  of  83.3  volumes,  composed  of 

CO2  =     14.4  per  cent. 


100.  0 

It  must  be  understood  that  equations  (36)  and  (37)  do  not 
represent  exactly  what  happens.  Gasoline  is  of  highly  complex 
composition,  rarely  containing  more  than  85  or  90  per  cent  of 
hexane,  the  remainder  being  compounds  of  uncertain  and  complex 
composition.  This  renders  exact  determinations  of  the  com- 
bustion reactions  almost  impossible.  It  is  highly  probable,  also, 
that  the  process  of  combustion  is  by  no  means  so  direct  as  the 
foregoing  equations  would  indicate.  They  are  given  here  merely 
as  the  basis  for  an  understanding  of  the  process  by  which  products 
of  combustion  are  formed.  They  serve  also  to  illustrate  the 
character  and  composition  of  the  exhaust  gases  from  complete 
and  incomplete  combustion. 

CHEMICAL  COMPOSITION  OF  AIR 
At  32°  F.  air  contains 

By  Weight  By  Volume 

Oxygen  ................................  23.6  per  cent.  21.3  per  cent. 

Nitrogen  ..............................  76.4    "  78.7    " 

100.0    "       "  100.0     "       " 

Therefore,  a  given  quantity  of  air  weighs 

100 

—  -  =  4.23  times  its  oxygen  content  (39) 

100 
or  —  —  =1.31  times  its  nitrogen  content.  (40) 

Similarly,  a  given  quantity  of  air  will  occupy 

—  =  4.69  times  the  volume  of  its  oxygen  '  (41) 


86  HANDBOOK  OF   CARBURETION 

or  -r—  =  1.27  times  the  volume  of  its  nitrogen.          (42) 

DETERMINATION  OF  AIR  NECESSARY  FOR  COMBUSTION 

The  amount  of  air  necessary  for  combustion  may  be  deter- 
mined as  follows: 

Let  it  be  required  to  determine  the  amount  of  air  necessary 
for  the  combustion  of  one  pound  of  carbon. 

First  write  the  combustion  equation 

C  +  02  =  C02  (43) 

Substitute  atomic  weights 

12  +  32  =  44  (44) 

Elements 
Divide  0  by  C 

^  =  2.66  Ibs.  of  O  (45) 

That  is,  i  pound  of  carbon  requires  2.66  pounds  of  oxygen  for 
its  complete  combustion.  As,  by  the  preceding  paragraphs,  air 
weighs  4.23  times  its  oxygen  content,  2.66  pounds  of  oxygen 
will  be  equivalent  to 

2.66  X  4.23  =  11.28  Ibs.  of  air. 

At  32°  F.  one  pound  of  air  occupies  12.387  cubic  feet,  so  that  the 
volume  of  11.28  pounds  of  air  will  be 

11.28  X  12.387  =  139.2  cu.  ft.  of  air. 

At  any  other  temperature  the  volume  will  be  proportional  to  the 
absolute  temperature 

VXT 

v  =  —j-  (46) 

Temperature  Correction 
Thus  at  90°  F.  the  volume  required  would  be 

'39-' X  (459 +  9°)  ft. 

(459  +  32) 


CHEMISTRY   OF   CARBURETION  87 


AIR  NECESSARY  FOR  COMBUSTION  FROM  ANALYSIS  OF  FUEL 

Compounds 

Owing  to  the  uncertainty  of  the  composition  of  hydrocarbon 
fuels,  it  is  frequently  convenient  to  determine  the  air  necessary 
for  combustion  from  an  ultimate  analysis  of  the  fuel.  This 
may  be  done  as  follows: 

Having  determined  the  percentage  composition  by  weight, 
this  percentage  is  expressed  as  a  decimal  when 


=  lbs.  O  per  pound  of  fuel.  (47) 

Thus,  an  analysis  of  a  standard  brand  of  gasoline  gave 

C  =  85.2  per  cent. 
H  =  14.8  per  cent. 

Substituting  these  values  in  equation  (47)  we  have 

(-^1  X  0.852)  +  (-y  X  0.148)  =  3.45  lbs.  of  O 
and 

-—2  =  14.6  lbs.  of  air  Ib.  of  fuel. 
.236 

Loss  FROM  INCOMPLETE  COMBUSTION 

Thermal  Losses 

It  is  thus  seen  that  unless  the  air/gas  ratio  is  at  least  14.6, 
incomplete  combustion  will  take  place  with  its  attendant  loss. 
This  loss  is  readily  understood  when  we  consider  that  i  pound 
of  carbon  burned  to  CO2  liberates  14,600  B.T.U.,  while  i  pound 
of  carbon  burned  to  CO  liberates  only  4,450  B.T.U.,  or  but  a 
little  better  than  30  per  cent  of  the  contained  heat.  The  loss 
thus  sustained  is  not  in  direct  proportion  to  the  CO  present, 
as  is  sometimes  stated,  but  is  rather  a  function  of  the  CO2/CO 


HANDBOOK   OF   CARBURETION 


ratio.  A  convenient  formula  is  given  by  Clerk  &  Burls  for  de- 
termining this  loss.  Slightly  modified  to  use  the  lower  heat 
value  of  the  fuel  this  formula  is 


o.7 


CL>2 

I-°3  +  TvT 


=  per'  cent  of  heat  lost. 


(48) 


CO 


There  are  certain  features  of  relative  throttle  opening  reducing 
compression  pressures,  with  resulting  diminution  of  fuel  efficiency 


iy 


CO, 


16        15        14        13 
Ratio  of  Air  to  Gasoline 


CHART  XV. 


Curves  plotted  from  tests  by  Professor  Watson,  which  show  the  relation 
between  the  products  of  combustion  and  the  ratios  of  air  to  gasoline. 

which  compensate  these  figures  slightly,  but  this  quantity  is 
quite  negligible  in  the  present  consideration. 

Chart  XV  shows  the  relationship  of  various  air/gas  ratios 
to  the  percentage  of  free  O2,  CO2,  and  CO  in  exhaust. 

Dangerous  Characteristics  of  Exhaust 

Beside  the  inefficiency  resulting  from  incomplete  combustion, 
there  are  other  disadvantages  in  having  carbon  monoxide  in 
the  exhaust  gas.  One  serious  consequence  which  may  result 
under  certain  conditions  is  the  possible  poisoning  of  persons 


CHEMISTRY   OF   CARBURETION  89 

who  inhale  the  gas  for  a  considerable  length  of  time.  CO  is  very 
poisonous  when  not  diluted  with  other  gases,  and  the  effect  is 
only  less  in  degree  when  it  forms  but  a  comparatively  small  pro- 
portion of  the  gas  inhaled.  The  evidence  of  poisoning  may  be 
nothing  worse  than  a  bad  headache,  but  persons  who  work  every 
day  in  ill- ventilated  garages,  the  atmosphere  of  which  is  seldom 
free  from  the  gas  exhausted  from  motors,  may  easily  suffer 
more  serious  consequences. 

Imperfect  combustion  is  also  the  cause  of  a  foul  smelling  and 
often  of  a  smoky  exhaust.  It  is  a  well  known  fact  that  an  over- 
rich  mixture  causes  black  smoke  from  this  cause. 

In  cases  where  the  exhaust  leaving  the  motor  contains  both 
oxygen  and  CO  as  a  result  of  poor  mixing,  the  combustion  may 
continue  in  the  exhaust  pipe  and  cause  the  latter  to  become 
excessively  hot.  This  overheating  often  results  in  scorching 
the  paint  on  parts  adjacent  to  the  exhaust  pipe  and  may,  under 
certain  conditions,  cause  a  serious  fire.  All  of  which  are  argu- 
ments in  favor  of  securing  the  most  complete  combustion  possible. 

This  overheating  of  the  exhaust  pipe  may  also  be  caused 
by  the  slow  burning  of  a  rich  mixture  which  causes  combustion 
to  be  continued  after  the  exhaust  valve  has  opened. 

The  same  slow  burning  in  a  lean  mixture  causes  combustion 
to  be  unfinished  even  upon  the  opening  of  the  intake  pipe  which 
ignites  the  charge  within  the  intake  manifold,  and  causes  back- 
firing, or  "popping"  as  it  is  called,  from  the  openings  of  the 
carbureter.  Danger  of  fire  from  this  phenomenon  can  be 
eliminated  by  placing  gauze  over  the  openings. 

Aside  from  these  considerations,  the  composition  of  the 
exhaust  may  be  taken  to  indicate  certain  very  definite  conditions 
of  carburetion.  The  following  rules  have  been  laid  down  and 
can  be  followed  without  error: 


Economic  Character  of  Exhaust 

I.  If  the  exhaust  contains  both  CO  and  O2  in  considerable 
quantities  (say  more  than  i  per  cent  of  each)  the  presumption  is 
that  the  gasoline  and  air  were  not  well  mixed,  either  because 


90  HANDBOOK   OF   CARBURETION 

of   inadequate   spraying   (deposition)    or   insufficient   heat  for 
vaporization. 

II.  If  CO  appears  in  the  exhaust  without  more  than  a  trace 
of  O2  being  present,  the  mixture  is  too  rich  and  the  supply  of 
gas  should  be  cut  down. 

III.  If  the  exhaust  contains  only  a  trace  of  either  or  both 
O2  and  CO,  the  balance  being  CO2,  the  combustion  is  complete — 
or  substantially  so.     Probably  a  slight  increase  in  the  air  will 
decrease  the  gasoline  per  horse-power  hour. 

IV.  If  the  exhaust  is  free  from  CO  and  contains  more  than 
4  per  cent  of  O2,  the  mixture  is  too  lean  and  more  gas  should  be 
admitted.     It  should  be  noted  that  the  curves,  Chart  XV,  indi- 
cate that  when  the  exhaust  contains  4  per  cent  of  02,  1 1  per  cent 
of  C02,  and  no  CO,  the  ratio  of  air  to  gas  is  17  to  i. 

DETERMINATION  OF  AIR/ GAS  RATIO 

One  of  the  most  important  uses  of  exhaust  gas  analysis  is 
for  the  determination  of  the  relative  proportions  of  fuel  and  air 
which  are  present  in  the  mixture.  That  this  can  be  very  closely 
approximated  from  the  composition  of  the  exhaust  gases  seems 
well  established.  Dr.  Watson's  curves,  shown  in  Chart  XV, 
are  available  only  when  the  exhaust  consists  of  either  C02  and 
O2  or  CO2  and  CO.  When  both  O2  and  CO  are  present,  the  air/ 
gas  ratio  may  be  determined  by  use  of  the  formula  given  by 
Clerk  and  Burls  in  "The  Gas,  Petrol,  and  Oil  Engine,"  Vol.  II, 
page  632,  as  follows: 


Air 
Corrected  ^ — 7  ratio  by  weight  = 


2.86  N 

(49) 


-£  »W    XI 

0.532  N  -  0.4  CO  -  2  (CO2  +  O2) 


In  this  formula  the  chemical  symbols  are  used  to  represent 
the  volume  per  cent  of  the  gases,  and  the  coefficients  are  based 
upon  an  analysis  of  the  fuel,  which  was,  in  the  case  cited,  C  = 
85.2  per  cent,  H  =  14.8  per  cent. 


CHEMISTRY   OF   CARBURETION  91 

Battantyne's  Constant 

In  determining  the  nitrogen  by  difference,  account  must  be 
taken  of  the  presence  of  free  H  and  CH4,  which  are  not  ordinarily- 
determined.  Ballantyne  has  shown,  however,  that  these  con- 
stituents bear  a  constant  ratio  to  the  percentage  of  CO  present 
in  the  following  proportions: 

Per  cent  of  free  H  =  0.36  per  cent  of  CO. 

Per  cent  of  CH4  =  0.12  per  cent  of  CO. 

On  page  631  of  the  same  volume  are  shown  comparative 
results  of  the  formula  from  which  the  foregoing  is  derived,  with 
results  of  actual  measurements  by  Dr.  Watson.  The  agreement 
is  sufficiently  close  for  all  practical  purposes,  particularly  if  a 
numerator  of  2.7  N  is  used  when  the  ratio  is  10  to  i  or  less. 

IMPORTANCE  OF  AIR/ GAS  RATIOS 

The  importance  of  the  air/gas  ratio  is  emphasized  in  some 
tests  recently  conducted  by  the  Automobile  Club  of  America.* 
Of  this  test  three  cars  ha^e  been  selected  for  purposes  of  illus- 
tration. All  three  cars  were  placed  under  strictly  test  conditions, 
so  far  as  it  was  possible  to  place  them  on  the  road.  Nine  samples 
of  exhaust  gases  were  taken  from  each  of  the  cars.  The  condi- 
tions of  motor  performance  during  the  taking  of  the  samples 
on  the  cars  were  as  follows: 

1.  Car   standing   after   motor   had   been    running    (motor 
running  at  low  speed). 

2.  When  car  was  accelerating  to  10  miles  per  hour  from  stand. 

3.  Car  running  10  miles  per  hour  on  level  on  second  or  third 
speed. 

4.  Car  running  15  miles  per  hour  on  level  on  top  speed. 

5.  Car  running  20  miles  per  hour  on  level  on  top  speed. 

6.  Car  running  30  miles  per  hour  on  level  on  top  speed. 

7.  Car  climbing  6  per  cent  gradient  at  20  miles  per  hour  on 
top  speed. 

8.  Car  climbing  5.75  per  cent  gradient  at  20  miles  per  hour  on 
second  or  third. 

*  The  Automobile,  Feb.  12,  1914. 


92 


HANDBOOK   OF   CARBURETION 


9.  Car  climbing  12.5  per  cent  gradient  at  20  miles  per  hour  on 
second  or  third. 

The  road  surface  while  taking  samples  3  to  6  was  wooden 
block.  Sample  7  was  taken  on  oiled  macadam,  8  on  smooth  wood 
block,  and  9  while  travelling  over  Belgian  block. 

The  results  of  analyses  are  as  follows: 


CAR  No.  i 

CAR  No.  2 

CAR  No.  3 

Test 

No. 

COi 

CO 

Oz 

C02 

CO 

02 

C02 

CO 

o. 

I 

II.  I 

2.4 

0.4 

9-1 

4-4 

1  .0 

5-2 

5.1 

4-2 

2 

13-6 

0.0 

0.6 

II.  I 

2-4 

0-3 

7.0 

4.6 

0.9 

3 

4 

ill 

0-3 
0.6 

10.6 

2-5 

O.2 

5-8 
5-3 

0.9 
0-5 

13-8 

1.5 

0.4 

9-2 

4-2 

0.2 

7-6 

4-3 

0.4 

6 

II.  8 

O.I 

0.2 

8.0 

4-3 

0.4 

6.8 

6.1 

0.6 

7 

12.9 

1.0 

0.0 

99 

3-6 

0-3 

7-4 

4-7 

0.4 

8 

13- 

0.8 

O.I 

6.6 

6.8 

0.4 

8.3 

3-6 

0-5 

9 

ii.  8 

2.0 

0.2 

6-4 

6.8 

0.4 

7-2 

4-4 

0.8 

Applying  formula  (49)  to  these  analyses  we  find  remarkable 
variations  in  the  mixtures,  not  only  in  different  cars  but  in  the 
same  car  under  different  test  conditions. 

These  results  are  plotted  in  Chart  XVI,  where  test  numbers 
are  plotted  as  abscissae  with  air/gas  ratios  as  ordinates. 

MAXIMUM  POWER  AND  MAXIMUM  THERMAL  EFFICIENCY 
Royal  Automobile  Club  Standard 

Dr.  Watson  (Proceedings  I.  A.  E.,  Vol.  Ill,  page  405)  has 
determined  that  maximum  power  is  developed  with  an  air/gas 
ratio  of  from  about  n  to  13,  while  maximum  thermal  efficiency 
occurs  with  a  ratio  of  about  17.  Hopkinson  and  Morse  (ibid., 
284),  show  that  maximum  thermal  efficiency  and  maximum 
power  occur  practically  together  at  a  ratio  of  about  14.  Ex- 
periments of  the  Massachusetts  Institute  of  Technology  show 
maximum  power  development  with  a  ratio  of  about  12,  which  is 
in  practical  agreement  with  Dr.  Watson's  results.  The  Royal 
Automobile  Club  has  decided  the  best  mixture  is  at  a  ratio 
of  14.5  as  giving  from  90  to  95  per  cent  of  both  thermal  efficiency 


CHEMISTRY    OF    CARBURETION 


93 


and  maximum  power.     This  is  in  reasonable  accord  with  the 
determinations  above  cited. 


Test  Number 


CHART  XVI. 


Advantages  of  a  Constant  Mixture 

Complete  combustion  is  possible  only  in  the  presence  of 
sufficient  air  in  intimate  admixture  with  the  fuel.  Maximum 
pressures  are  obtainable  only  within  very  narrow  limits  of 
mixture  composition.  Both  are  essential  to  efficiency. 

Chart  XVII  is  plotted  from  a  tabulation  of  experiments  of 
the  Massachusetts  Institute  of  Technology.  The  time  in  sec- 
onds required  for  the  explosion  pressure  to  reach  its  maximum 


94 


HANDBOOK   OF   CARBURETION 


is  plotted  against  air/gas   ratios  by  weight.    The  maximum 

pressure  in  pounds  per  square  inch  appears  against  each  point. 

Inspection  of  this  chart  shows  that  the  greatest  pressures  are 

obtained  when  the  rate  of  burning  is  fastest,  and  that  departure 


67° 

/ 

/ 

/ 

s~ 

15 

14 
°1? 

/ 

>< 

/ 

°76 

/ 

Z 
3 

7 

=  85 

Li 

1C  of  Q 

\  -Uost 

3u  ruing 

£ 

s 

78^ 

9 
8 

83° 

\ 

^ 

^ 

\ 

62 

'"> 

*• 

.06         .07         .08         .09         .10         .11         .12 
Time  (Seconds)  required  to  reach 
Maximum  Pressi  re 

CHART  XVII. 

from  the  line  of  quickest  burning  either  toward  richness  or  lean- 
ness means  a  rapid  falling  off  in  power.  It  is  true  that  by  a 
proper  spark  advance  this  loss  of  power  may  be  compensated 
for  to  some  extent,  but  even  an  automatic  spark  control  would 
have  to  be  nimble  to  follow  the  varying  ratios  shown  in 
Chart  XVI. 


CHEMISTRY   OF   CARBURETION 


95 


Relative  Volumes  of  Exhaust 

When  an  excess  of  air  is  present  in  the  mixture  with  gasoline 
vapor  there  is  but  a  small  increase  in  the  final  volume  of  the 
exhaust,  but  when  the  fuel  is  in  excess  the  volume  increase  is 


1.150 


•3  1.100 

> 

2  1.075 


1.050 


17          16          15          14          13 
Lbs.  of  Air  to  Lb.  of  Gasoline 

CHART  XVIII. 


quite  large.     The  graph  of  Chart  XVIII,  from  Dr.  Watson 
(Cantor  Lectures,  1910),  shows  this  increase  in  volume  ratios. 

Rich  Mixtures 

It  will  be  noted  that  with  air/gas  ratios  from  14  to  the  limit 
of  combustibility,  the  change  is  small,  but  that  from  14  down 
the  increase  is  comparatively  rapid.  Thus  with  a  10  to  i 
mixture,  the  volume  of  the  exhaust,  reduced  to  the  original 
temperature  and  pressure,  would  have  increased  nearly  14  per 
cent,  indicating  a  substantial  gain  in  power  from  this  cause, 
but  at  the  expense  of  a  loss  of  heat  shown  in  equation  (48). 

Furthermore,  the  gain  by  increased  volume  is  offset  by  the 
reduction  of  explosion  pressures  of  rich  mixtures,  as  shown 
in  Chart  XVII.  In  some  engine  designs  there  undoubtedly  is  a 
final  small  gain  in  maximum  power  output  afforded  by  enriched 
mixtures,  provided  ignition  can  be  properly  timed.  It  is  doubtful 
however,  if  any  pronounced  accelerative  effect  is  commonly 


96  HANDBOOK   OF   CARBURETIOX 

produced  by  sudden  over-enrichment  of  the  mixture  because  a 
favorable  combination  of  the  foregoing  conditions  is  of  rare 
occurrence. 

This  common  misconception  doubtless  arises  from  the  fact 
that  many  carbureters  have  a  tendenc>  toward  impoverishment  of 
the  mixture  upon  the  sudden  opening  of  the  throttle.  In  such  a 
device  it  is  probable  that  the  actual  "enrichment  for  accelera- 
tion" is  not  really  as  greatly  in  excess  of  the  normal  mixture  as  is 
commonly  supposed. 

Lean  Mixture 

On  the  other  hand,  a  lean  mixture  entails  similar  losses 
without  a  proportional  compensation  of  increased  volume. 
Hence  it  is  seen  that  cutting  down  the  fuel  does  not  necessarily 
mean  economy,  because,  owing  to  reduced  pressure  and  volume, 
a  greater  quantity  of  mixture  is  necessary  to  obtain  a  given  road 
speed. 

The  lines  of  maximum  thermal  efficiency,  maximum  power, 
with  some  sacrifice  of  fuel  economy,  and  the  Royal  Automobile 
Club  standard,  have  been  plotted  in  Chart  XVI,  and  even  casual 
inspection  of  the  diagram  will  show  how  far  the  cars  under  test 
departed  from  ideal  conditions.  The  reason  for  the  relative  fuel 
mileages  of  the  cars  is  also  apparent.  The  diagram  also  shows 
the  erratic  carbureter  action  to  which  the  engines  were  subjected. 

For  instance,  had  the  carbureter  of  Car  No.  i  maintained 
throughout  the  test  anything  approaching  the  constancy  it 
exhibited  in  tests  2  and  3,  its  fuel  record,  already  good,  would 
have  been  greatly  improved.  Had  the  carbureter  of  Car  No.  3 
maintained  its  same  constancy  with  decreased  fuel,  this  car 
would  have,  in  all  likelihood,  surpassed  the  performance  of  Car 
No.  i  bo'th  in  fuel  mileage  and  general  smoothness  of  operation. 

Loss  From  Imperfect  Combustion 

Applying  formula  (48)  to  the  analyses,  we  find  fuel  losses  as 
direct  as  if  the  gasoline  tank  had  been  opened  and  its  contents 
allowed  to  waste,  as  follows: 

Car  No.  i — 6.7  per  cent  loss. 


CHEMISTRY   OF   CARBURETION  97 

Car  No.  2 — 23.0  per  cent  loss. 
Car  No.  3 — 30.0  per  cent  loss. 

Free  O2  and  CO 

The  foregoing  losses  are  the  direct  result  of  the  presence  of 
CO  and  exist  because  of  it.  CO  furthermore  is,  ordinarily,  an 
indication  of  an  over-rich  mixture  with  all  the  losses  that  con- 
dition entails.  This  is  not  always  the  case,  however,  particu- 
larly when  CO  is  present  in  small  quantities,  and  even  more 
obviously  when  it  is  associated  with  free  02.  The  latter  condi- 
tion has  excited  much  scientific  speculation.  That  it  is  due 
largely,  but  not  wholly,  to  imperfect  contact  of  the  molecules 
of  fuel  and  air,  the  writer  has  demonstrated  to  his  entire  satis- 
faction. Claims  have  been  made  that  liquid  fuel  particles 
actually  passed  through  the  cylinders  unburned  or  but  partially 
burned.  This  is  more  difficult  of  credence,  but  not  impossible. 
There  is,  however,  one  theory  which  seems  to  have  been  gener- 
ally overlooked,  but  which,  if  it  is  ever  established,  will  demand 
serious  consideration  on  the  part  of  the  designer.  Some  years 
ago  MM.  Mallard  and  LeChatelier  demonstrated  in  a  glass 
container  that  during  a  certain  phase  of  concussive  flame  propa- 
gation, the  flame  was  extinguished  before  combustion  was  com- 
plete. This  they  attributed  to  an  action  not  unlike  the  echo 
of  a  sound  wave.  The  vibratory  character  of  flame  propagation 
through  an  explosive  mixture  is  commonly  accepted,  and  it 
would  seem  possible  that  only  a  slight  accentuation  would  be 
necessary  to  cause  vibrations  which  might  extinguish  the  flame. 
A  careful  study  of  this  phenomenon  might  lead  to  distinct 
progress. 

However,  there  is  at  present  no  better  way  of  obtaining 
knowledge  of  the  thermal  and  chemical  reactions  taking  place 
within  the  gas  engine  cylinder  than  through  the  medium  of 
exhaust  gas  analysis.  That  the  results  of  gas  analyses  have 
seemed  inconsistent  at  times  is  due  rather  to  improper  inter- 
pretation of  results  than  to  any  inherent  fault  in  the  results 
themselves. 


98 


HANDBOOK   OF   CARBURETION 

A  METHOD  OF  ANALYSIS 


Sampling 

One  of  the  chief  reasons  for  apparently  erratic  results  is  that 
the  analysis  is  performed  upon  samples  not  fairly  representative 


FIG.  20. 


of  the  true  composition  of  the  exhaust  gas.  Accuracy  depends 
fully  as  much  on  the  method  of  taking  the  sample  as  upon  the 
detail  of  the  analysis  itself. 


CHEMISTRY   OF   CARBURETION  99 

The  method  of  inserting  a  tube  into  the  discharge  end  of  the 
exhaust  pipe  is  not  to  be  countenanced.  Between  the  pulsations 
of  the  exhaust  is  a  period  of  diminished  or  even  sometimes  sub- 
atmospheric  pressure.  When  this  exists,  air  may  be  actually 
drawn  into  the  exhaust  pipe  for  a  considerable  distance,  with 
consequent  vitiation  of  any  sample  taken  under  these  conditions. 

The  method  of  tapping  a  sample  pipe  into  the  exhaust  pipe 
between  the  engine  and  the  muffler  is  also  inaccurate,  even 
though  the  inner  end  of  the  sample  tube  be  directed  against  the 
exhaust  pressures.  With  such  a  device  the  sample  is  taken 
only  of  the  center,  or  core,  of  the  exhaust  stream,  as  shown 
diagramniatically  at  S,  Fig.  19,  or  if  the  pipe  be  not  bent,  only 
the  surface  of  the  stream  will  be  sampled  as  at  S,  Fig.  20. 


FIG.  21. 

The  most  accurate  location  of  the  sample  pipe  seems  to  be  as 
shown  diagrammatically  in  Fig.  21. 

In  this  arrangement  the  sample  pipe  T  is  inserted  through  the 
entire  diameter  of  the  exhaust  pipe  E.  The  portion  within  the 
exhaust  pipe  5  is  cut  longitudinally  so  as  to  present  an  opening 
toward  the  flowing  stream,  across  the  entire  diameter  of  the 
exhaust  pipe.  A  sample  withdrawn  through  this  arrangement 
is  fairly  representative  of  the  entire  stream  of  the  exhaust.  Its 


100  HANDBOOK   OF   CARBURETION 

use  will  be  found  to  give  invariably  consistent  results,  provided 
other  conditions  are  properly  met. 

Leaky  Exhaust  Pipes 

The  exhaust  pipes  of  very  few  cars  will  be  found  wholly 
free  from  air  leaks,  due  to  defective  gaskets,  poor  threads,  or 
even  piping  rendered  porous  by  rus.t.  The  slightest  leak  is,  of 
course,  fatal  to  the  accuracy  of  the  sample.  An  excellent 
method  to  determine  whether  an  exhaust  line  is  tight  is  to  set 
the  carbureter  adjustments  so  that  far  too  rich  a  mixture  is 
delivered  to  the  cylinders.  If  the  analysis  of  the  exhaust  shows 
even  >£  per  cent  of  free  oxygen,  it  is  conclusive  proof  that  there 
is  an  air  leakage  and  the  same  should  be  corrected  before  pro- 
ceeding further. 

Collecting  the  Sample 

Having  determined  freedom  from  air  leaks,  the  sample  is 
taken  by  connecting  the  collecting  tube,  Fig.  22,  with  the  sample 
pipe  by  means  of  a  rubber  tube. 

The  collecting  tube  is  first  filled  with  water,  slightly  acidu- 
lated with  sulphuric  acid  to  prevent  the  possible  existence 


FIG.  22.    GAS  COLLECTING  TUBE. 

of  free  alkalies  which  would  also  absorb  part  of  the  C02  of  the 
collecting  sample.  The  collecting  tube  is  then  held  in  a  vertical 
position  and  the  upper  stop-cock  opened  wide.  The  lower  stop- 
cock is  then  opened,  allowing  the  exhaust  gas  to  replace  the 
water  which  flows  to  waste. 

ACTUAL  ANALYSIS 

The  desired  number  of  samples  having  been  collected,  each 
in  turn  is  next  transferred  to  an  Orsat  apparatus,  Fig.  23. 


CHEMISTRY   OF   CARBURETION 


101 


The  burette  P  holds  100  cubic  centimeters,  and  is  graduated 
in  i/io  c.c.  It  is  filled  with  distilled  water  by  elevating  the 
bottle  B.  One  end  of  the  collecting  tube  is  connected  by  means 
of  a  rubber  tube  to  A.  The  other  end  of  the  collecting  tube 
is  connected  to  the  water  supply  under  slight  pressure.  The 


FIG.  23.    ORSAT  APPARATUS. 

bottle  B  is  placed  below  the  burette  as  at  B,  and  the  stop-cock 
on  the  tube  is  opened.  Upon  opening  the  stop-cocks  on  the 
connecting  tubes  the  gas  will  flow  into  the  burette  P,  displacing 
the  water  therein  into  the  bottle  B. 

More  than  100  c.c.  are  so  withdrawn,  all  stop-cocks  are 
closed,  and  the  collecting  tube  disconnected. 

The  cock  A  is  now  opened  and  the  level  carefully  brought 
to  zero  by  slightly  elevating  the  bottle  B  and  opening  its  stop-cock. 

When  zero  is  reached,  exactly  100  c.c.  of  gas  will  be  contained 
in  the  burette  P,  and  the  cock  on  A  should  be  closed. 


102  HANDBOOK  OF  CARBURETION 

Determination  of  CO2  and  02 

The  pipette  No.  i  contains  a  solution  of  potassium  hydrate 
KOH.  Into  this  the  entire  volume  of  gas  is  now  passed  by 
opening  the  stop-cocks  on  pipette  and  bottle,  and  elevating  the 
latter.  The  gas  is  passed  in  and  out  of  this  several  times  and 
finally  withdrawn  into  P  until  the  KOH  solution  in  No.  i  stands 
at  the  mark  on  the  neck  where  it  stood  originally.  It  will  now  be 
found  that  the  water  in  P  no  longer  stands  at  zero.  This  is 
because  the  KOH  solution  has  absorbed  the  CO2  present.  Con- 
sequently the  reading  of  the  burette  is  the  volume  percentage  of 
C02  that  has  been  absorbed.  Before  taking  this  reading,  one 
minute  should  be  allowed  for  all  drainage  to  take  place  from  the 
walls  of  the  apparatus,  otherwise  the  reading  may  be  too  low. 
Before  any  reading  is  accepted  as  final,  the  process  should  be 
repeated  until  two  coincident  readings  are  obtained.  When 
this  occurs  the  manipulation  is  repeated,  this  time  passing 
the  gas  into  pipette  No.  2,  which  contains  a  solution  of  potassium 
pyrogallate.  This  solution  absorbs  oxygen,  and  its  percentage 
is  read  on  the  burette  as  before,  first  deducting  the  C02  previ- 
ously determined  from  the  total  burette  reading. 

Determination  of  CO 

Again  the  process  is  repeated,  this  time  into  burette  No.  3, 
containing  a  solution  of  cuprous  chloride,  which  absorbs  CO. 
This  completed,  the  gas  in  the  burette  consists  mainly  of  nitrogen, 
with  small  percentages  of  free  hydrogen  and  marsh  gas.  There 
being  no  convenient  method  of  determining  these  gases,  we  are 
obliged  to  estimate  their  percentage  by  Ballantyne's  constant, 
noted  in  connection  with  equation  (49).  Their  quantity  is  small 
and  their  constancy  to  the  amount  of  CO  present  sufficiently 
permanent,  so  that  the  possible  error  caused  by  their  non- 
determination  is  negligible. 

It  should  be  noted  that  while  the  absorption  of  CO2  is  very 
rapid,  the  second  reading  usually  checking  the  first,  the  absorp- 
tion of  O2  and  CO  is  much  slower,  requiring  many  transfers  for 
its  accomplishment.  With  the  average  gas,  a  complete  and 
accurate  analysis  usually  requires  about  forty  minutes. 


CHAPTER  VII 
THE  PHYSICAL  CONDITIONS  OF  CARBURETION 

No  LESS  important  than  the  chemical  composition  of  a  mixture 
is  its  physical  condition.  Certain  aspects  of  this  subject  have 
been  treated  of  in  the  chapter  on  Intake  Manifolds,  but  its 
importance  warrants  a  more  thorough  study. 

As  has  been  shown,  the  functions  of  carburetion  are  dual. 
Not  only  must  the  fuel  be  mixed  with  a  definite  amount  of  air, 
but,  to  be  effective,  the  fuel  must  be  absorbed  by  the  air.  Air 
being  a  gas,  no  absorption  of  fuel  can  take  place  until  it,  too, 
becomes  a  gas. 

HEAT 

This  involves  the  absorption  of  heat  for  two  purposes:  First, 
to  raise  the  temperature  of  the  fuel  to  the  evaporation  point; 
and  second,  to  supply  the  heat  absorbed  by  actual  vaporization. 

Specific  Heat 

The  British. thermal  units  necessary  to  raise  the  temperature 
of  one  pound  of  a  substance  one  degree  Fahrenheit  is  called  the 
Specific  Heat  of  the  Substance. 

Latent  Heat 

The  heat  absorbed  without  change  of  temperature  during  a 
change  of  state,  as  from  a  solid  to  a  liquid  or  from  a  liquid  to  a 
gas,  is  called  the  latent  heat. 

Gasoline,  as  it  is  called  in  this  country,  is  of  uncertain  chemi- 
cal composition,  and  its  specific  heat  and  latent  heat  are 
therefore  uncertain.  We  quote  the  following  from  leading 
authorities : 

103 


104 


HANDBOOK   OF   CARBURETION 


TABLE    II. 
TABLE  OF  LATENT  AND  SPECIFIC  HEATS  OF  PETROLEUM  PRODUCTS 


Product 

Specific 
Heat 

Temperature 

Latent 
Heat 

Authority 

Petroleum  

0.511 

21  °  to  58°  C 

Pagliana 

Petroleum  

0.498 

1  8°  to  99°  C 

Pagliana 

Crude  Petroleum, 

japan... 

0-453 

Mabery  and 

Goldstein 

Crude  Petroleum, 

Penn  

o  soo 

Goldstein 

Crude  Petroleum, 

California.  . 

w.  jv-nj 
0.398 

Goldstein 

Crude  Petroleum, 

Russia  .... 

0-453 

Goldstein 

Kerosene  Sp.  Gr. 
Kerosene  Sp.  Gr. 

810  
811  

0.499 
0.470 

"  260°  "F*' 

105-4 

Redwood 
Robinson 

Naphtha,  Sp.  Gr. 
Naphtha,  Sp.  Gr. 

•756  
.720  

0.510 
0.569 

175°     F* 
115°     F* 

103-5 
100.6 

Redwood 
Redwood 

Gasoline,  Sp.  Gr. 

642.  .  . 

0.580 

70°     F* 

100.2 

Redwood 

Petroleum  ether  .  . 

0-445 

100°     C 

Eckerlein 

Petroleum  ether  .  . 

0.419 

o°     C 

Eckerlein 

Benzol  (C6H6)  .    .  . 

0.407 

10°     C 

Pickering 

Benzol  (C6H6)  .    .  . 

0.450 

50°     C 

Pickering 

Benzol  (C6H6)  .    .  . 

0.482 

65°     C 

Deruyts 

Benzol  (C6H6)  .    .  . 

o°     C 

109. 

Regnault 

Benzol  (C6H6)  .    .  . 

80.  i  C 

92.9 

Wirtz 

Benzol  (C6H6)  .    .  . 

8o.35C 

93-5 

Schiff 

*  Boiling-points. 

From  the  foregoing  table  it  is  seen  that  averages,  sufficiently 
close  for  all  practical  purposes,  may  be  assumed  to  be 
Specific  heat  =      o.$cv  B.T.U. 
Latent  heat   =  100.0      B.T.U. 

Specific  and  Latent  Heat  of  Gasoline 

On  no  subject  connected  with  gasoline  as  a  fuel  does  there 
seem  to  be  such  a  divergence  of  views  as  upon  the  latent  heat. 
Table  II  is  a  compilation  from  authorities  quoted  by  the 
U.  S.  Bureau  of  Standards,  supplemented  by  other  recent 
authorities. 

Total  Heat 

Latent  heat  must  not  be  confused  with  Total  Heat,  which  is 
the  heat  necessary  to  raise  the  temperature  of  a  substance  to  a 
given  degree,  plus  the  latent  heat  of  vaporization  at  that 
temperature. 

The  subject  of  the  specific  and  latent  heats  of  fuel  is  of 
primary  importance  in  practical  carbureter  design.  It  is  highly 


PHYSICAL   CONDITIONS   OF   CARBURETION  105 

probable  that  the  lack  of  definite  knowledge  concerning  these 
factors  has  seriously  retarded  the  use  of  heavier  fuels.  Further- 
more, a  more  complete  grasp  may  be  obtained  of  problems  of 
starting,  water-jacketing,  and  the  like  by  a  thorough  under- 
standing of  the  action  of  heat. 

Reduction  of  Temperature  by  Evaporation 

Considering  first  the  temperature  drop  occasioned  by  the 
evaporation  of  gasoline,  let  us  assume  that  the  carbureter  is 
delivering  a  mixture  of  i  part  of  gasoline  to  15  parts  of  air  by 
weight.  Let  us  assume  the  temperature  of  the  air,  and  con- 
sequently that  of  the  gasoline,  to  be  60°  F. 

The  specific  heat  of  air  at  constant  pressure  is  0.2375,  there- 
fore the  total  heat  available  for  each  degree  drop  in  temperature 
is  (i  X  .500)  +  (15  X  .2375)  =  3.0625  B.T.U.  But  as  the  heat 
necessary  to  vaporize  i  pound  of  gasoline  is  100  B.T.U.,  the 
resulting  temperature  drop  will  be 

100 


and  the  resulting  temperature  in  the  carbureter  or  manifold 
will  be 

60  -  32.4  =  27.6°  F. 

Frost-Covered  Manifolds 

This  accounts  for  the  appearance  of  frost  on  the  manifold  until 
the  temperature  beneath  the  hood  raises  sufficiently  to  supply  the 
necessary  heat  by  the  conductivity  of  the  walls  of  the  manifold. 

Effect  of  Mixture  Proportion 

The  richer  the  mixture,  the  greater  the  temperature  drop, 
provided  the  fuel  is  vaporized,  and  the  fact  that  some  manifolds 
do  not  indicate  this  marked  temperature  drop  is  proof  that  a 
large  percentage  of  the  fuel  is  carried  to  the  cylinder  still  in  the 
form  of  a  liquid. 


106  HANDBOOK   OF   CARBURETION 

Every  liquid  fuel  has  a  definite  temperature  below  which  no 
inflammable  vapor  is  given  off.  With  the  commercial  gasoline 
of  the  present  day,  this  temperature  is  increasing  as  the  gravity 
decreases.  It  is  too  uncertain  to  fix  a  definite  value,  but  the 
difficulty  of  starting  a  cold  engine  is  attributable  directly  to 
the  reduction  of  temperature  within  the  carbureter  below  this 
critical  point. 

Flash-Point 

An  illustration  of  this  is  furnished  by  an  attempt  to  start 
with  kerosene  mixed  with  gasoline.  The  vaporization  tem- 
perature, or  "flash-point,"  of  kerosene  is  upward  of  80°  F.  If 
the  temperature  drop  is  32°,  as  noted,  the  initial  temperature  of 
both  air  and  fuel  must  be  at  least  80  +  32,  or  112°  F.,  for  com- 
plete evaporation.  This  is  recognized  in  kerosene  carbureters, 
which  commonly  start  the  engine  on  the  more  volatile  gasoline, 
continuing  the  operation  with  kerosene  only  when  the  necessary 
heat  has  been  supplied  by  the  engine. 

Necessity  for  Artificial  Heat 
In  a  15  to  i  mixture 

i5  X  .2375  =  2.56  B.T.U. 

must  be  supplied  to  the  air  for  each  pound  degree  rise  of  temper- 
ature, while  .5  B.T.U.  is  supplied  to  the  fuel.  Consequently, 
in  the  use  of  heavier  fuels  at  least,  the  necessity  for  pre-heating 
the  air  is  apparent. 

Loss  of  Volumetric  Efficiency  by  Heat 

It  is  frequently  urged  against  this  practice  that  the  weight 
of  mixture  entering  the  cylinder  is  decreased  by  heat  with  con- 
sequent loss  of  power.  The  extent  of  this  loss  of  volumetric 
efficiency  may  be  determined  by  a  consideration  of  the  expansion 
of  gases  by  heat. 

According  to  the  law  of  perfect  gases 

PV 

-T  =  A,  a  constant, 


PHYSICAL   CONDITIONS    OF   CARBURETION  107 

when 

P  —  pressure. 

V  =  volume. 

T  =  absolute  temperature. 

At  32°  F.  a  cubic  foot  of  dry  air  at  sea  level  weighs  0.080728 
pounds.    The  volume  of  i  pound  is,  therefore,  — « «  =  12.387 

cubic  feet.     The  pressure  per  square  foot  is  2116.2  pounds. 
PV       2116. 2  X  12.387 

T-        491.13  '     =53'37 

If  the  temperature  of  a  gas  is  raised  i°  F.  (from  32°  to  33°  F.), 
we  find  that  the  volume  of  i  pound  is 

RT      T/    53-37  X492-*3 

T"  =  V  2116.2          =  I2'411  cublc  feet 

That  is,  for  a  rise  in  temperature  of  each  i°  F.,  the  volume  of  a 
given  weight  will  be  increased 

12.387 

^i  -  •OOI96 

or  practically  2/IO  of  i  per  cent. 

If,  therefore,  air  is  admitted  to  the  carbureter  at,  say  100°  F. 
above  the  atmospheric  temperature,  the  volumetric  loss  from 
heating  would  be  about 

0.2  (100°  —  32.4°)  =  13.5  per  cent 
when  all  the  fuel  is  vaporized  before  reaching  the  cylinder. 

Partial  Vaporization 

It  is  evident  that  if  but  partial  vaporization  takes  place  the 
reduction  of  the  temperature  will  be  less,  and  the  volumetric 
loss  proportionately  more.  This  is  a  strong  argument  in  favor 
of  complete  evaporation  of  the  fuel  before  it  reaches  the  cylinder 
of  a  four-stroke  cycle  engine.  With  the  two-stroke  cycle,  evapo- 
ration is  usually  completed  by  the  temperature  and  mechanical 


108  HANDBOOK  OF  CARBURETION 

agitation  within  the  crank  case,  so  that  the  charge  is  delivered 
to  the  combustion  chamber  at  fully  reduced  temperature. 

Effect  of  Vaporization  Within  the  Cylinder 

It  has  been  stated  that  with  some  four-stroke  cycle  marine 
engines,  evaporation  of  liquid  fuel  particles  within  the  cylinder 
during  the  compression  stroke  causes  an  increase  of  power  by  a 
reduction  of  the  energy  expended  in  compression.  The  evidence 
submitted  points  to  the  accuracy  of  this  observation,  but  it  is 
doubtful  if  it  is  the  direct  result  of  an  actual  reduction  of  temper- 
ature. Lower  temperature  means  lower  compression  pressure, 
which  in  turn  implies  reduced  efficiency  and  power  output.  It 
seems  probable  that,  if  the  claim  is  correct,  it  is  due  to  heat 
transference  from  the  hot  walls,  restoring  or  even  increasing  the 
normal  compression  temperature. 

Economy  of  Artificial  Heat 

Certain  it  is  that  heat  is  desirable  with  the  present  grades  of 
fuel.  This  is  indicated  by  the  greater  mileage  secured  by  recent 
systems  of  fuel  feed,  wherein  an  appreciable  quantity  of  fuel  is 
held  in  a  sheet  metal  reservoir  in  proximity  to,  and  receiving 
the  heat  from,  the  engine.  The  heat  necessary  to  raise  the  fuel  to 
its  evaporation  point,  and  also  at  least  a  portion  of  the  heat  neces- 
sary for  evaporation,  are  secured  in  this  way.  Under  these  con- 
ditions, vaporization  is  more  complete  and  greater  efficiency  is 
the  direct  result. 

STARTING 

In  starting  an  engine  when  none  but  atmospheric  heat  is 
available,  but  one  alternative  remains  when  temperatures  are 
low.  A  fuel  mist  must  be  introduced  into  the  cylinders  and  these 
liquid  particles  ignited  and  burned  until  the  temperature  raises 
sufficiently  to  supply  heat  for  the  gasification  of  the  fuel,  either 
from  a  water-jacket,  heated  air  from  around  the  exhaust  pipe, 
or  by  radiation  and  conduction  from  the  engine  itself. 


PHYSICAL   CONDITIONS   OF   CARBURETION  10!) 


"Rich  Mixture  for  Starting" 

Such  an  understanding  of  actual  conditions  shows  the  utter 
fallacy  of  the  oft  reiterated  statement  that  "a  rich  mixture  is 
necessary  for  starting."  The  fact  is  that  under  ordinary  con- 
ditions, no  gaseous  mixture  whatever  is  delivered  to  the  cylinders, 
or  at  best  but  a  mixture  containing  so  little  Juel  gas  as  to  be 
uninflammable.  Such  portion  of  the  liquid  fuel  as  is  not  de- 
posited in  carbureter  and  manifold  reaches  the  combustion 
chamber  as  liquid  particles  more  or  less  finely  divided.  In  order 
to  ignite  at  all,  there  must  be  far  more  of  these  liquid  particles 
present  than  would  be  necessary  were  they  evaporated.  When 
a  true  gas  fuel  is  delivered  to  the  cylinders  (as  in  the  case  of  an 
engine  using  illuminating  gas  as  a  fuel)  there  is  no  trouble  with 
starting  at  any  temperature  without  changing  the  mixture 
proportions  from  those  of  continued  running.  Hence  the  state- 
ment that  "a  rich  mixture  is  necessary"  should  become  "an 
excess  of  liquid  fuel  is  required  for  starting."  Such  an  ex- 
pression would  help  materially  to  remove  one  of  the  stumbling- 
blocks  from  the  path  of  carbureter  development. 

EFFECT  OF  TEMPERATURE  ON  FUEL  FLOW 
Viscosity  of  Gasoline 

The  property  of  viscosity  is  not  ordinarily  associated  with 
liquids  as  light  as  gasoline.  It  is  a  fact,  however,  that  the  flow 
of  gasoline  through  the  nozzle  of  a  carbureter  is  directly  affected 
by  changes  of  temperature.  Chart  XIX  shows  the  results  of 
measurements  of  flow  at  temperatures  between  50°  and  100°  F. 
It  will  be  noted  that  at  100°  F.  the  discharge  of  a  nozzle  is  about 
36  per  cent  greater  than  at  50°  F. 

It  is  thus  seen  that  a  carbureter  nozzle  adjusted  to  give  a 
proper  mixture  at  a  working  temperature  of  100°  F.  will  discharge 
but  a  little  over  71  per  cent,  of  the  requisite  fuel  when  the  tem- 
perature falls  to  50°  F.,  which,  as  has  been  shown,  is  the  very 
time  when  an  excess  of  fuel  is  needed. 


110 


HANDBOOK   OF   CARBURETION 


Like  other  characteristics  of  gasoline,  its  viscosity  varies 
with  its  composition,  therefore  definite  regulation  of  the  fuel 
orifice,  thermostatic  or  otherwise,  is  consequently  difficult. 


300' 


90 


50 


1.2 
Relative  Flow 

CHART  XIX. 


PRESSURE 
Variation  of  Compression  Pressures 

At  practical  sea  level,  the  normal  pressure  of  the  atmosphere 
is  29.92  inches  of  mercury,  equivalent  to  14.7  pounds  per  square 
inch.  The  maximum  variation  is  about  3  inches,  i.e.,  from  28  to 
31  inches,  about  10  per  cent.  The  density  or  weight  of  a  cubic 
foot  of  air  varies  inversely  as  the  pressure,  hence  when  the 
barometer  stands  at  31  inches,  10  per  cent  more  weight  of 
mixture  should  be  delivered  to  the  cylinders  than  during  periods 
of  extreme  low  pressures. 


PHYSICAL   CONDITIONS   OF   CARBURETION  111 

Effect  of  Reduced  Pressures  on  the  Auxiliary  Valve 

What  is  commonly  called  the  "vacuum"  in  a  carbureter 
is  really  an  expression  of  the  pressure  difference  between  the 
interior  of  the  carbureter  and  the  outside  atmosphere.  Lower 
pressures  mean  lower  pressure  differences,  or,  as  it  is  commonly 
called,  "less  vacuum."  This  causes  the  auxiliary  air- valve  to 
open  a  lesser  amount.  Now  at  a  given  engine  speed  the  same 
volume  of  air  (at  reduced  density)  is  drawn  into  the  cylinders 
and,  as  the  air- valve  is  opened  a  lesser  amount,  the  total  ad- 
mission area  is  decreased.  This  entails  a  higher  velocity  over 
the  fuel  jet.  This  increase  of  velocity  is  sufficiently  high  to 
more  than  compensate  for  the  decreased  density  (see  equation  5) 
and,  as  a  result,  too  much  fuel  is  inspirated.  Thus,  unless  a  car- 
bureter is  governed  by  velocity,  independently  of  other  pressure 
differences,  it  will  be  susceptible  to  barometric  changes. 

Effect  of  Altitude  on  Compression  Pressures 

This  effect  is  particularly  noticeable  at  higher  altitudes 
when,  if  adjustments  are  not  necessary,  it  is  simply  an  indication 
that  the  engine  had  previously  been  running  at  lower  altitudes 
or  with  an  inefficient  mixture. 

Effect  of  Altitude  on  Vaporization 

Chart  XX  shows  the  barometric  pressure  at  different  alti- 
tudes, and  also  the  effect  of  the  diminished  pressure  on  the 
boiling-point  of  water.  This  evaporative  effect  of  reduced 
pressures  is,  of  course,  even  more  pronounced  on  liquids  of  lighter 
gravity,  such  as  gasoline,  so  that  vaporization  is  more  complete 
at  higher  altitudes.  As  noted  in  a  previous  paragraph,  this 
occasions  a  correspondingly  greater  temperature  drop,  and  hence 
winter  starting  at  high  altitudes  is  usually  more  difficult  than  at 
sea  level,  at  the  same  temperature. 

It  is  common  to  experience  a  noticeable  lack  of  power  at 
high  altitudes.  This  may  be  due  to  no  fault  of  the  carbureter, 
after  the  latter  is  properly  adjusted  to  meet  the  new  conditions. 


112 


HANDBOOK   OF   CARBURETION 


Inches  of  Mercury 
SS         8 


§         8 


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3000 

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Fahrenheit  Degrees 
CHART  XX. 


PHYSICAL   CONDITIONS    OF   CARBURETION  113 

Air  Standard  of  Efficiency 

The  efficiency  of  an  internal  combustion  engine  is  a  direct 
function  of  the  compression.     For  any  gas  engine  where 

r  =  compression  ratio, 
Cp  =  specific  heat  at  constant  pressure, 
Cv  =  specific  heat  at  constant  volume, 
E  =  efficiency, 

Cp 

r~c^ 

and 


which  shows  that  high  efficiencies  depend  upon  high  compression. 

Effect  of  Altitude  on  Power 

The  volumetric  efficiency  of  compression  decreases  approx- 
imately 3  per  cent  with  each  thousand  feet  of  altitude,  so  that  at 
10,000  feet  we  find  a  reduction  in  compression  of  about  30  per 
cent.  The  loss  of  power  entailed  by  this  reduction  in  compression 
can  be,  even  partially,  compensated  by  the  carbureter,  only  in 
the  event  that  the  mixture,  originally  too  poor,  becomes  enriched 
from  causes  already  mentioned.  Because  this  combination  of 
circumstances  sometimes  happens  in  touring,  claims  are  set 
forth  that  the  carbureter  used  is  insusceptible  to  barometric 
changes.  Such  claims  lead  to  confusion  which  only  retards 
practical  development.  Usually,  however,  the  effect  of  such  an 
altitude  is  to  so  vitiate  the  proportions  of  the  mixture  as  to  make 
readjustments  imperative.  If  this  is  made  so  that  a  slightly 
richer  mixture  than  normal  is  obtained,  it  is  possible  to  com- 
pensate for  a  considerable  portion  of  the  volumetric  loss. 


CHAPTER  VIII 

THE  CARBURETER  OF  THE  FUTURE 

Balanced  Forces 

CAREFUL  study  of  the  problems  of  carburetion  indicates 
certain  definite  relations  between  the  various  forces  employed. 
The  ideal  carbureter,  if  it  ever  arrives,  will  express  the  delicate 
balance  between  these  forces  in  its  design. 

Changing  Fuel 

The  chief  difficulty  to  be  surmounted  lies  in  the  uncertain 
and  ever-varying  composition  of  the  fuel.  It  has  been  true  in 
the  past  that  the  most  efficient  carbureter  would  become  rela- 
tively useless  owing  to  changes  in  composition  of  commercial 
fuel.  There  are  good  reasons  for  believing  that  this  condition 
exists  to-day  to  a  far  less  extent.  A  carbureter  adapted  to 
handle  the  fuels  of  the  present  day  will  employ  principles  which 
will  be  found  commercially  operative  with  lower  grades  of  fuel 
than  are  likely  to  be  forced  into  general  use  for  many  years. 

Constancy  of  Mixture 

The  ideal  carbureter  design  at  present  indicated  seems  to 
necessitate  the  embodiment  of  ten  salient  features. 

1.  Mixture  composition  must  be  automatically  maintained 
at  any  desired  air /gas  ratio,  irrespective  of  speed  changes,  load 
or  weather  conditions.     Neither  accelerating  nor  idling  should 
change  the  mixture  proportions,  but  if  such  constancy  is  un- 
attainable the  tendency  should  be  toward  enrichment  rather 
than  impoverishment  of  the  mixture. 

Atomization 

2.  High  velocities  must  be  employed,  at  least  over  the  fuel 
nozzle,  in  order  to  produce  the  fine  atomization  necessary  for 
efficient  vaporization. 

114 


THE  CARBURETER  OF  THE  FUTURE  115 


Velocities 

Particularly  should  the  velocity  at  cranking  speed  be  high. 
At  this  time  no  heat  is  available  for  vaporization,  and  conse- 
quently whatever  fuel  gas  is  found  must  be  the  direct  result  of 
decreased  pressures  acting  on  the  greatest  surface  possible,  a 
condition  synonymous  with  high  velocity. 

At  high  speeds  the  velocity  may,  in  fact  should,  decrease  in 
order  to  obtain  the  extreme  limit  of  volumetric  efficiency.  At 
high  speeds  much  heat  is  generated  and  this  may  be  utilized  to 
produce  the  necessary  gasification  of  the  fuel. 

Whether  the  jet  should  assume  the  form  of  a  single  nozzle  or 
many,  a  slot,  an  annulus,  or  other  form,  is  a  matter  for  experi- 
mental determination. 

Size  and  Shape  of  Passages 

3.  Passages  for  the  flow  of  both  air  and  gasoline  should 
conform,  so  far  as  possible,  to  the  natural  shape  of  the  vena 
contracta.  The  venturi-shaped  passage  makes  for  constancy  of 
the  coefficient  of  flow,  and  therefore  permits  of  more  accurate 
determination  of  dimensions.  Unnecessary  bends  should  be 
avoided,  particularly  in  all  air  passages,  and  sharp  corners  or 
baffling  projections  should  not  be  tolerated.  Passages  should 
be  made  with  sufficient  cross-sectional  area  not  to  exert  undue 
wire-drawing,  or  throttling  of  the  charge.  In  brief,  the  internal 
resistance  of  the  instrument  should  be  at  the  minimum. 


Application  of  Heat 

4.  The  application  of  the  heat  to  the  instrument  should  be 
made  with  full  recognition  of  the  thermal  reactions  involved. 
If  error  is  made,  it  should  be  toward  the  side  of  excess  heat,  for, 
notwithstanding  the  loss  of  volumetric  efficiency  involved,  this 
is  always  partially  and  sometimes  wholly  overcome  by  the 
greater  rate  of  flame  propagation,  with  the  consequent  rise  of 
explosion  pressure,  which  is  induced  by  a  higher  initial  temper- 


116  HANDBOOK   OF   CARBURETION 

ature  of  the  charge.     Here  again  must  the  balance  be  established 
in  conformity,  to  a  certain  extent,  with  engine  design. 

Water- Jacketing 

Lacking  definite  knowledge  of  conditions  and  still  to  design  a 
universal  carbureter,  it  is  well  to  provide  means  for  regulating 
the  temperature  of  the  heated  air,  as  by  an  adjustable  vent  in 
the  hot  air  pipe.  Regulation  of  the  temperature  of  jacket  water 
is  less  necessary,  because,  in  ordinary  design,  the  passage  of  the 
air  through  the  carbureter  is  of  such  short  duration  that  little 
heat  is  absorbed  by  the  air.  The  water-jacket  serves  only  to 
supply  heat  for  the  vaporization  of  deposited  liquid  fuel  and 
because  of  the  reduction  of  temperature  incident  to  this  proc- 
ess, there  is  little  danger  of  raising  the  temperature  of  the  air 
unduly.  The  jacketing,  however,  should  be  confined  to  the 
walls  of  such  passages  as  cause  deposition  by  their  enlargement 
of  the  cross-sectional  area  and  consequent  reduction  of  the 
velocity. 

Adjustments 

5.  Adjustments,  however,  should  be  minimized,  if  not 
abolished  altogether.  It  will  take  many  years  of  education  to 
convince  the  truck-driver  that  he  does  not  know  more  about  his 
carbureter  than  the  designer,  and  until  that  Utopian  day  arrives 
the  engineer  can  protect  himself,  his  reputation,  and  his  product 
only  by  removing  every  adjustment  which  he  considers  not 
absolutely  vital. 

Fuel  Level 

What  adjustments  are  essential  will  depend  upon  the  type 
and  design  of  the  instrument,  but  it  seems  certain  that  many  of 
these  at  present  in  use  can  be  abolished.  For  instance,  inex- 
perienced operators  seem  to  delight  in  readjusting  the  level 
in  the  fuel  reservoir,  notwithstanding  that  its  practical  effects 
on  the  mixture  are  wholly  negligible.  By  equation  (13)  it  may 
readily  be  determined  that  a  difference  of  even  one-half  inch 
gives  rise  to  an  inappreciable  error  even  at  the  highest  velocities. 


THE  CARBURETER  OF  THE  FUTURE  117 


Moving  Parts 

6.  Moving  parts  should  be  abolished  as  far  as  possible. 
What  remain  should  be  so  constructed  as  to  not  change  their 
functions  even  after  the  normal  wear  to  which  every  mechanism 
is  subject. 

No  moving  part  should  have  a  sliding  fit  in  the  direction  of 
the  air-current.  Dust  and  dirt  are  liable  to  render  such  a  device 
inoperative.  Undue  wear,  or  even  constant  wear,  must  be 
provided  against,  and  to  this  end  movements  should  be  small  in 
extent  and  intermittent  in  character. 

Accessibility 

7.  Accessibility  should  be  a  prime  consideration.    The  design 
should  be  such  that  complete  disassembly  can  take  place  without 
removing    the    carbureter    from    the    manifold.     The    gasoline 
nozzle  and  all  its  passages  should  be  particularly  accessible,  for, 
despite  any  filtering  devices,  dirt  at  times  persists  in  selecting 
the  fuel  nozzle  as  a  resting  place.    The  float  reservoir,  too,  should 
be  specially  accessible  and  should  be  provided  with  a  convenient 
drain  whereby  an  accumulation  of  water  and  other  impurities 
may  be  occasionally  removed. 

Priming 

8.  The  carbureter  should  be  provided  with  some  device  for 
supplying  an  excess  of  fuel  when  cold.     This  device  should  be 
automatic  in  its  action  but  must  be  wholly  inoperative  except 
at  relatively  low  temperatures,  otherwise  it  would  function  every 
time  the  engine  was  slowed  to  cranking  speed.     The  result  would 
be  too  rich  a  mixture  at  low  speed,  entailing  a  useless  waste  of 
fuel,  carbon  deposits  in  the  cyh'nder,  and  general  unsatisfactory 
action. 

Fire  Protection 

9.  If  means  are  ever  found  whereby  the  fuel  flow  is  t  auto- 
matically compensated  for  temperature,  danger  from  fire  will  be 


118  HANDBOOK   OF    CARBURETION 

practically  unknown.  Until  then,  openings  should  be  protected 
with  wire  screen  sufficiently  heavy  to  afford  the  necessary  cooling 
but  not  of  so  fine  a  mesh  as  to  become  readily  clogged  and  thus 
prevent  the  admission  of  the  proper  amount  of  air. 

Practical  Manufacture 

10.  Finally,  the  whole  must  be  embraced  in  a  design  capable 
of  the  most  advanced  manufacturing  methods.  Interchange- 
ability  of  parts  is  an  intensely  practical  requirement  to  manu- 
facturer and  user  alike:  to  the  first,  because  of  reduced  manu- 
facturing cost;  to  the  latter,  because  in  the  event  of  accident 
he  is  certain  of  prompt  replacement. 

Summary 

Summarized,  there  is  no  reason  why  the  carbureter  should 
not  become  as  standard  and  reliable  a  product  as  the  engine 
itself.  Its  functions  are  in  reality  far  less  involved,  and  the 
avowed  idiosyncrasies  of  the  carbureter  of  to-day  have  existence 
only  in  our  lack  of  knowledge  concerning  the  principles  of 
carburetion.  The  day  cannot  be  far  distant  when  an  efficient 
combination  of  capital  and  engineering  skill  will  solve  the  re- 
maining problems,  thereby  increasing  automobile  efficiency  in 
the  broadest  sense  of  the  term,  by  bringing  carbureter  troubles 
to  an  end. 


APPENDIX 

USEFUL  TABLES  AND   CONVENIENT  FORMULA 

i.    VACUUM  GAUGE 
To  Find  Absolute  Pressures  Shown  by 

Let  R  =  vacuum  gauge  reading, 
B  =  barometer  reading, 
P  =  absolute  pressure, 


then 


(B  -  R)  .49  =  P. 


2.     MERCURY  COLUMNS 


i  inch  mercury  weighs  .49131  pounds  per  square  inch.    (Log 

1-6913557.) 

i  inch  mercury  =  13.647  inch  H2O.     (Log  1.1350532.) 

3.     COMPRESSION  EFFICIENCY  AT  DIFFERENT  ALTITUDES 

TABLE  III. 

For  air  at  yo-pound  gauge  pressure.      (Hiscox.) 


Feet  Above  Sea 
Level 

Volumetric  Efficiency 
of  Compression, 
Per  Cent 

Loss  in 
Capacity, 
Per  Cent 

Decreased  Power 
Required, 
Per  Cent 

O 

100. 

0. 

0. 

IOOO 

97- 

3- 

1.8 

2OOO 

93- 

7- 

3-5 

3000 

90. 

10. 

5-2 

4OOO 

87. 

13- 

6.9 

5000 

84. 

16. 

8-5 

6OOO 

81. 

19- 

IO.I 

7OOO 

78. 

22. 

11.  6 

8000 

76. 

24. 

I3-I 

9000 

73- 

27. 

14.6 

10000 

70. 

30- 

16.1 

IIOOO 

68. 

32. 

17.6 

12000 

65- 

35- 

19.1 

13000 

63- 

37- 

20.6 

14000 

60. 

40. 

22.1 

15000                                     58. 

42. 

23-5 

NOTE. — For  pressures  above  70  pounds  gauge,  deduct  3  per  cent  from  the  figures  in  column 
2  above,  and  10  per  cent  from  the  figures  in  the  last  column,  for  each  10  pounds  increase  above 
70  pounds  (approximate). 

119 


120  HANDBOOK  OF  CARBURETION 

4.     To  REDUCE  HEAD  IN  FEET  TO  PRESSURE  IN 
POUNDS  PER  SQUARE  INCH 

Let     p  —  pressure  of  atmosphere  in  pounds  per  square  inch  . 
P  =  pressure  of  atmosphere  in  pounds  per  square  foot 


H  =  head  in  feet  necessary  to  cause  atmospheric  pressure,/?. 
h  =  head  in  feet  necessary  to  cause  pressure  of  i  pound. 
Wa  =  weight  of  i  cubic  foot  of  air. 

then  Wa  =  H 


and 


ff  =  Pounds  Per  square  inch  for  i-foot  head. 


Thus,  if          p    =  14.7 

Wa  =      .076 

14.7  X  144 


=  27816 
076  ' 


27816 

and  —  =  1802.22 

14.7 


or  O~A  =  -00052847  pounds  per  square  inch  for  i-foot  head. 

5.    To  DETERMINE  DROP  IN  PRESSURE  BY  VELOCITY 
Let 

h  =  head  in  feet  to  cause  i  pound  pressure  = =  1892.22  ft. 

14.7 

v  =  velocity  in  feet  per  second. 
p  =  drop  in  pressure. 


USEFUL  TABLES  AND  CONVENIENT  FORMULA      121 

6.    VELOCITY  or  FLOW 

Let 

v  =  velocity  of  flow  in  feet  per  second. 
p  =  pressure  causing  the  flow  in  pounds  per  square  inch. 
g  =  acceleration  of  gravity  32.2  feet  per  second  per  second. 
h  =  head  in  feet  necessary  to  cause  a  pressure  of  i  pound  = 

1892.22  feet. 
c  =  coefficient  of  flow. 

then 


2gh 

consequently  v~  =  2ghp  X  c 

whence  v   =  c  >  2  gh  ^  p 

or  v   =  348.87  c  *p 

7.    MANOMETER  PRESSURES 

i  inch  water       =      .036  pounds. 

i  inch  mercury  =      .491  pounds. 

i  inch  mercury  =  13.647 
27.8  inch  water       =     i  pound. 
38.6  inch  gasoline  specific  gravity  0.72  =  i  pound. 
2.4  inch  mercury  =     i  pound. 

8.    DISPLACEMENT 
Let 

D  =  displacement  of  cylinders  in  cubic  inches. 
Ve  =  volumetric  efficiency. 

g  =  gear  ratio. 
M  =  miles  per  hour. 

d  =  diameter  drive-wheels  in  inches. 
A  =  area. 

5  =  stroke  in  inches. 

n  =  number  of  cylinders. 

b  =  bore  in  inches. 


122  HANDBOOK  OF  CARBURETION 

R  =  r.p.m.  of  engine. 

D  =  b2  .7854  sn. 


_  3456  cubic  feet  per  minute 
RX  Ve 

_  1 20  cubic  inches  per  second 
RX  Ve 


Engine  speed,  R  =  33 


DVe 

Cubic  feet  per  revolution,        = 


Cubic  feet  per  minute 
Cubic  inches  per  second 


3456 
DRVe 
3456 
DRVe 


-....  2.8  gMDVe 

Cubic  inches  per  second  — - — 

DRVe 
Velocity  in  feet  per  second      = — 

DgMVe 
Velocity  in  feet  per  second       =  — - — r 

4.2857.4 

Car  speed  in  miles  per  hour,  M  =  — 

336s 

9.    MILES  PER  HOUR  TO  FEET  PER  SECOND 

5280     X  M.P.H.  =  feet  per  second 
60  X  60 

or 

1.4666  M.P.H.  =  feet  per  second  (Log  0.1663304). 


USEFUL   TABLES   AND   CONVENIENT  FORMULA  123 

10.    To  DETERMINE  VOLUME  RATIOS   FROM  WEIGHT  RATIOS 

Let 

Wf  =  weight  i  cubic  foot  fuel  vapor. 
A  =  air /fuel  ratio  by  weight. 
Wa  =  weight  of  i  cubic  foot  of  air. 

R  =  air/fuel  ratio  by  volume, 
then 

Wf  X  A 

Wa 
and 

—  =  per  cent  of  fuel  by  volume. 
J\. 

ii.    ACCELERATION  COMPUTATIONS 
Let 

vf  =  initial  velocity  in  feet  per  second. 
»"=  final  velocity  in  feet  per  second. 
a  =  acceleration  in  feet  per  second  per  second. 
/  =  time  in  seconds, 
then 

v'  =  v"  —  at 


v"  - 

a  =  — 


a 
12.     COMPUTATIONS  OF  VELOCITY 

Cubic    ft./min.  X  28.8  =  cubic    inches    per    second    (Log 

I-4593925)- 

Velocity2  in  feet    per    second  X   .0x3022735  =  inches  water 
(Log  4.3567071). 

Inches  water  X  4398.3  =  velocity  in  ft. /sec.  (Log  3. 643 2 g 29). 


124  HANDBOOK   OF   CARBURETION 

13.    Loss  OF  PRESSURE  IN  PIPES  (KENT) 

Let 

p  =  pressure  loss  in  pounds  per  square  inch. 

v  =  velocity  of  air/feet  per  second. 

L  =  length  of  pipe  in  feet. 

d  =  diameter  of  pipe  in  inches. 

Lv* 

P    =    O.OOOOO25   —r- 


»    =    632-5\   Y 

.0000025  Lv2 


14.    EFFECT  OF  BENDS  (KENT) 

Radius  of  bend 

in  diameters  of  pipe,  532      i^      1^4         i      ^       % 
Equivalent  lengths 

of  straight  pipe 

diameters,  7.85  8.24  9.03  10.36  12.72  17.51  35.09  121.2 

15.    WATER 

Weighs  62.355  pounds  per  cubic  foot. 
i  foot  head  =  0.433  pounds  per  square  inch. 
i  inch  head  =  0.0360860  per  square  inch. 
i  pound  pressure  =  2.306  feet  head. 
13.647  inches  =  i  inch  of  mercury. 

1  6.    VOLUMETRIC  EFFICIENCY 

Actual  cubic  feet  per  minute 
Displacement  in  cubic  feet  per  minute  =  Volumetnc   efficiency- 


Let 


then 


USEFUL  TABLES  AND  CONVENIENT  FORMULAE 
17.    BRAKE  HORSE-POWER 

5  =  r.p.m. 

T  =  scale  reading  in  pounds. 
r  =  radius  of  brake-arm. 


125 


ST27TT 

33000 


B.H.P. 


18.     CAPACITY  OF  PRONY  BRAKES 

Each  square  foot  of  rim  surface  of  a  water-cooled  iron  brake 
pulley  will  absorb  10  B.H.P.  without  undue  heating. 


19.    FORMULAE  FOR  TEMPERATURE  CORRECTION  FOR  SPECIFIC 


Let 


then 


GRAVITY  OF  GASOLINE 

S  =  specific  gravity  at  60°  F. 
s  =  specific  gravity  at  f  F. 
t  =  temperature  in  F.° 


I  —  .0007  (t  —  60) 


TABLE  IV. 

20.    WEIGHT  OF  GASES  AT  32°  F.  AND 
INCHES  MERCURY  (Kent) 


29.92 


Pounds  per 
Cubic  Feet 

Cubic  Feet 
per  Pound 

Air  
Hydrogen 

0.080728 
O  OOS^Q 

12.388 

178  o^i 

Oxygen  
Nitrogen  
Carbon  monoxide  
Carbon  dioxide  

0.08921 
0.07831 
0.07807 
0.12267 

11.209 
12.770 
12.810 
8  152 

120 


HANDBOOK  OF  CARBURETION 


TABLE  V 

21.     BAUME'S  HYDROMETER  AND  CORRESPONDING 
SPECIFIC  GRAVITY  (Kent) 

Formula.     Specific  Gravity  =  140  -r-  (130  +  degrees  B) 


Degrees  Batime 

Specific  Gravity 

Degrees  Baume 

Specific  Gravity 

10.  0 

1.  000 

32.0 

0.864 

II.  0 

0.993 

33-0 

0.859 

12.0 

0.986 

34-0 

0.854          , 

13-0 
14.0 

0.979 
0.972 

35  .0 
36.0 

0.849 
0.843 

15.0 

16.0 

0.966 
0.959 

37  o 
38.0 

0.838 
0-833 

17.0 

0.952 

39  o 

0.828 

18.0 

0.946 

40.0 

0.824 

19.0 

0.940 

41.0 

0.819 

20.0 

0-933 

42.0 

0.814 

21.0 

0.927 

44.0 

0.805 

22.0 

0.921 

46.0 

0.796 

23.0 

0.915 

48.0 

0.787 

24.0 

0.909 

50.0 

0-7/8 

25-0 
26.0 

0.903 
0.897 

52.0 
54-o 

0.769 
0.761 

27.0 

0.892 

56.0 

0-753 

28.0 

0.886 

58.0 

0-745 

29.0 

0.881 

60.0 

0-737 

30.0 

0.875 

65  o 

0.718 

31.0 

0.870  . 

70.0 

0.700 

75-0 

0.683 

22.    BRITISH  THERMAL  UNIT 

i  B.T.U.  =  the  amount  of  heat  necessary  to  raise  i  pound  of 
water  from  62°  to  63°  F. 

NOTE. — It  takes  slightly  more  than  i  B.T.U.  to  raise  I  pound  of  water  i°  F.  above  63°  F. 
and  slightly  less  below  62°  F.,  but  these  quantities  are  so  small  as  to  be  negligible  in  practice. 

i  calorie  =  3968  B.T.U.  =  i  kilog.  of  water  i°  C. 

i  B.T.U.  =  778  foot-pounds  of  work  =  Joule's  Equivalent. 

42.42  B.T.U.  per  minute  =  i  horse-power. 

2545  B.T.U.  per  hour  =  i  horse-power. 

To  Find  B.T.U.  Equivalent  to  any  Rise  in  Temperature 
Rise  in  F.°  X  weight  X  5  =  B.T.U. 

To  Find  the  Rise  in  Temperature  by  tfte  Addition  of  a  Given 
Number  of  B.T.U. 
B.T.U. 


weight  X  S 


=  Rise. 


USEFUL   TABLES    AND    CONVENIENT   FORMULAE 


127 


when  5  =  specific  heat  (q.  v.). 

TABLE  VI 
23.  VOLUME,  PRESSURE,  AND  DENSITY  OF  AIR 

From  a  normal  volume  and  pressure  of  62°  Fahr.     (Haswell.) 


F. 

Volume 
of  a  Pound 

Absolute  Pressure 
of  a  Constant 
Volume  of  Heat 

Density  or  Weight 
of  i  Cubic  Foot  of 
Free  Air 

0 

1  1  •  583 

12.96 

.086331 

32 

12.387 

13-86 

.080728 

40 

12.586 

14.08 

•079439 

12.840 

14.36 

.077884 

62 

13    HI 

14.70 

.076097 

70 

13.342 

14.92 

.074950 

80 

13.593 

15.21 

•073565 

90 

13.845 

15-49 

.  072230 

100 

14.096 

15-77 

.  070942 

1  20 

I4.592 

16.33 

.068500 

140 

15.100 

16.89 

.066221 

160 

15   603 

17.50 

.  064088 

1  80 

16.  106 

18.02 

.062090 

200 

16.606 

18.58 

.060210 

2IO 

16.860 

18.86 

•059313 

212 

16.910 

18.92 

•059135 

22O 

17.111 

19.14 

.058442 

240 

17.612 

19.70 

•  056774 

260 

18.116 

20.27 

•  055200 

280 

18.621 

20.83 

.053710 

300 

19.121 

21.39 

•  052297 

320 

19.624 

21-95 

•050959 

340 

20.  126 

•22.51 

.049686 

360 

20.630 

23.08 

.048476 

380 

21.131 

23-64 

•  047323 

400 

21.634 

24.20 

.  046223 

425 

22.262 

24.90 

.  044920 

450 

22  .  890 

25.61 

.043686 

475 

23.518 

26.31 

.042520 

500 

24.146 

27.01 

.041414 

525 

24-775 

27.71 

.040364 

550 

25-403 

28.42 

•039365 

575 

26.031 

29.12 

•038415 

600 

26.659 

29.82 

.037510 

650 

27-9I5 

31.23 

•  035822 

700 

29.171 

32.635 

.  034280 

800 

3I.68I 

35  •  445 

.031561 

900 

34-197 

38.255 

.029242 

IOOO 

36.8II 

41.065 

.027241 

2OOO 

61  .  940 

69.165 

.016172 

3000 

87.130 

97.265 

.011499 

INDEX 

Absolute  pressures  by  vacuum  gauge,  119 
Acceleration,  37,  43,  44,  47,  63,  64,  123 
effect  of  inertia  of  moving  parts  on,  10 
measurements  of,  43,  44,  63 
measurements  of,  on  the  block,  37 
of  gravity,  1 
Accelerometer,  the,  41 
Accessibility,  117 

Adjustments,  undesirability  of,  116 
Air  admission  area,  15 

flow,  desirable  passages  for,  115 
effect  of  bends  on,  23 
formuLe  for,  1,  6,  13,  15,  77 
gas  ratio,  determination  of,  90 

ratio,  importance  of,  91 
measurements  by  orifice  in  thin  plate,  76 

by  Venturi  meter,  80 
standard  of  efficiency,  113 
valve,  true  function  of,  6 

weighted,  10 

velocity  of,  in  terms  of  fuel  flow,  15 
volume  pressure  and  density  of,  127 
Altitude,  effect  of,  on  atmospheric  pressure,  112 
of,  on  compression  pressures,  111,  119 
of,  on  power,  113 
of,  on  vaporization,  6,  111 
Analysis,  method  of  gas,  98 
Anemometer,  the,  76 

Apparatus  for  carbureter  measurements,  78 
determining  fuel  consumption,  39 
gas  analysis,  Orsat,  101 
sampling  for  gas  analysis,  98 
Atomization,  26,  114 

Automobile  Club  of  America,  tests  by  gas  analysis,  91 
Auxiliary  valve,  effect  of  reduced  pressure  on,  111 
failure  of  corrective  devices,  7 
inherent  error  of  the,  8 
true  functions  of  the,  6 
weighted,  10 

Ballantyne's  constant,  91 

Baume  hydrometer  and  corresponding  specific  gravities,  126 

129 


130  INDEX 

Bends,  resistance  of,  to  air  flow,  31,  124 

Boiling-point  of  water,  table  of,  112 

Brake  horse-power,  determination  of,  35,  48,  61,  125 

mean  effective  pressure,  81 

Prony,  capacity  of,  125 
British  thermal  unit,  126 

Calorie,  126 

Capacity  of  Prony  brake,  125 

Carbon  dioxide,  determination  of,  102 

monoxide,  determination  of,  102 

presence  of  in  the  exhaust,  97 
Carbureter,  compensating,  5 

constant  vacuum,  10 

governed  by  velocity,  13 

multiple  jet,  8 

of  the  future,  114 

simple,  4 

testing  on  the  block,  35 

with  compensating  nozzle,  11 

variable  fuel  orifice,  9 
Carburetion,  physical  conditions  of,  103 

within  the  manifold,  24 
Car  speed,  formula  for,  122  • 
Characteristics,  disclosure  of,  by  test,  70 
Chart  I. — Volumetric  loss  by  velocity,  22 
Chart  II. — Results  by  accelerometer,  46 
Chart  III. — Results  by  accelerometer,  49 
Chart  IV. — Rolling  resistance  by  dynamometer,  54 
Chart  V. — Complete  results  by  traction  drum  measurements,  60 
Chart  VI. — Results  of  individual  car  tests,  63 
Chart  VII.— Results  of  individual  car  tests,  65 
Chart  VIII.— Results  of  individual  car  tests,  66 
Chart  IX. — Results  of  individual  car  tests,  67 
Chart  X. — Results  of  individual  car  tests,  69 
Chart  XI. — Results  of  individual  car  tests,  70 
Chart  XII. — Results  of  individual  car  tests,  71 
Chart  XIII.— Results  of  Riedler's  tests,  72 

Chart  XIV, — Comparison  of  performance  of  six  representative  American  cars,  73 
Chart  XV. — Relation  of  products  of  combustion  to  air/gas  ratios,  88 
Chart  XVI. — Air/ gas  ratios  of  three  cars  on  the  road,  93 
Chart  XVII. — M.  I.  T.  experiments  on  rate  of  flame  propagation,  94 
Chart  XVIII.— Relative  volumes  of  exhaust,  95 
Chart  XIX. — Effect  of  temperature  on  gasoline  flow,  110 
Chart  XX. — Effect  of  altitude  on  pressure  and  boiling-point  of  water,  112 
Chemical  composition  of  air,  85 
of  exhaust  gases,  88 

reactions  of  combustion,  84 


INDEX  131 


Chemistry  of  carburetion,  83 
Combustion,  84 

determination  of  air  necessary  for,  86 

incomplete,  loss  from,  87,  96 
Comparing  performances,  39,  48,  73,  93 
Comparison  of  six  American  cars,  73 

test  stand  and  road  results,  68 

with  Dr.  Riedler's  methods,  74 
Compensating  carbureter,  5 

nozzle,  11 

Compensation  by  velocities,  13 
Composition  of  air,  85 

exhaust  gases,  85,  88 

Compression,  efficiency  of,  at  altitudes,  119 
Condensation  in  the  manifold,  22 
Constancy  of  velocities,  17 
Constant  mixture,  17,  93,  114 

Corrective  devices  for  the  auxiliary  air  valve,  failure  of,  7 
Cubic  feet  per  minute  to  cubic  inches  per  second,  123 

Danger  from  the  exhaust,  88,  117 

Density,  volume  and  pressure  of  air,  127 

Deposition  in  the  manifold,  22 

Detailed  investigation,  possibilities  of,  74         ,  . 

Determination  of  acceleration,  37,  43,  44,  47,  63,  123 

air  flow  by  orifice  in  thin  plate,  76 

air  flow  by  Venturi  meter,  80 

air/ gas  ratio,  90 

ah*  necessary  for  combustion,  86 

air  per  revolution,  122 

brake  horse-power,  48,  61,  125 

brake  mean  effective  pressure,  81 

carbon  dioxide,  102 

carbon  monoxide,  102 

carbureter  action,  76 

car  speed,  122 

draw-bar  pull,  47,  56 

drop  in  pressure  by  velocity,  120 

engine  friction,  45 

engine  speed,  122 

flexibility,  38 

hill-climbing  ability,  63 

indicated  horse-power,  48 

losses  from  incomplete  combustion,  87 

maximum  horse-power,  35 

oxygen,  102 

performance  with  fixed  load,  36 

reduction  of  temperature  by  evaporation,  105 


132  INDEX 

Determination  of  retardation,  44 

rolling  resistance,  45,  54 

speed  range,  64 

thermal  efficiency,  49 

total  resistance,  45 

transmission  friction,  46 

velocity,  122 
Diffusion,  27 

Displacement  formulae,  121 
Distribution,  32 

qualitative,  20,  27 
quantitative,  20,  31 
Drainage  of  manifolds,  32 
Draw-bar  pull,  47,  56,  62 

vs.  horse-power,  75 
Durley's  formula  of  flow,  77 
Dynamometer  for  recording  rolling  resistance,  54 

Economizers,  27 
Effect  of  bends,  31,  124 

leaky  exhaust  pipes  on  gas  analysis,  100 
lean  mixtures,  96 
rich  mixtures,  95 
Efficiency,  air  standard  of,  113 

of  compression  at  altitudes,  111,  119 
Engine  friction,  determination  of,  45 

speed,  formula  for,  122 
Error  of  the  auxiliary  valve,  8 
Evaporation,  effect  of,  on  temperature,  105 

partial,  effect  of,  107 

within  the  cylinder,  108 
Exhaust,  danger  from,  88,  117 

economic  characteristics  of  the,  89 

gas  analysis,  availability  of,  83 

pipe,  leaky,  effect  of,  on  gas  analysis,  100 

relative  volumes  of  the,  84,  95 
Explosion  pressures,  time  element  of,  93 

Flame  propagation,  rate  of,  94 
vibratory  extinction  of,  97 
Flash  point,  106 
Fluid  flow,  law  of,  1 
Formula  for  absolute  pressures  by  vacuum  gauge,  119 

acceleration,  43,  62,  64,  123 

air  admission  areas,  15 

air  standard  of  efficiency,  113 

barometric  and  temperature  correction  of  the  Venturi  meter,  81 

brake  horse-power,  62,  125 


INDEX  133 

Formula  for  brake  mean  effective  pressure,  49 

car  speed,  122 

determination  of  air/ gas  ratios,  90 

determination  of  air  quantities  by  orifice  in  thin  plate,  79 

displacement,  122 

draw-bar  pull,  47,  62 

drop  in  pressure  by  velocity,  120 

effect  of  temperature  on  fuel  flow,  3 

engine  speed,  122 

flow  of  fluids,  1,  77 

flow  of  fuel,  3 

grade,  64 

heat  lost  by  incomplete  combustion,  88 

loss  of  pressure  in  pipes,  124 

miles  per  hour  to  feet  per  second,  122 

net  effective  power,  64 

rolling  resistance,  62 

spring  deflection,  15 

temperature  correction  of  specific  gravity  of  gasoline,  125 

temperature  rise  with  given  B.  T.  U.,  126 

thermal  efficiency,  50 

vacuum  in  inches  of  water,  15 

velocity  of  air  in  terms  of  fuel  velocity,  15 

velocity  of  flow,  121,  122 

volume  ratios  from  weight  ratios,  123 
Form  of  test  report,  61 
Friction,  engine,  determination  of,  45 

transmission,  determination  of,  46 
Frost  covered  manifolds,  105 

Fuel,  apparatus  for  determining  consumption  of,  39,  57 
check  on  car  speed  limit,  66 
consumption,  65 
deposition  of,  22 

effect  of  temperature  on  the  density  of,  3 
formula  for  flow  of,  3 
level,  effect  of,  116 
orifice,  variable,  9 
Function  of  the  auxiliary  air-valve,  6 

Gas  analysis,  method  of,  98 

Orsat  apparatus  for,  101 
Gases,  weight  of,  125 
Gasoline,  composition  of,  84 

temperature  correction  for  specific  gravity  of,  125 

viscosity  of,  109 

weight  of,  121 
Grade,  formula  for,  64 


134  INDEX 

Hard  starting,  causes  of,  24 
Head,  definition  of,  1 

in  feet,  to  pressure  in  Ibs.,  120 
Heat,  economy  of,  108 

desirable  conditions  of,  115 

latent,  103 

necessity  for,  25,  106 

specific,  103 

Heating  the  carbureter  and  manifold,  25 
Hexane,  chemical  reactions  of,  84 
Hill-climbing  ability,  63 
Horse-power,  determination  of,  35,  48,  61 
Hydrogen  in  the  exhaust,  91 

Inadequacy  of  block  testing,  39 
Inches  of  water  to  velocity  in  feet  per  second,  123 
Incomplete  combustion,  losses  from,  87,  96 
Intake  manifold,  areas  of  the,  26 

deposition  in,  22 

functions  of,  20 

length  of,  27 

the,  20 

velocities  in,  22 
Investigation  of  details,  possibilities  of,  74 

Jacketing  the  manifold,  25,  30,  33 
Joule's  equivalent,  126 

Latent  heat,  103 

of  petroleum  products,  104 
Law  of  perfect  gases,  106 
Lean  mixture,  effect  of,  96 
Loss  from  incomplete  combustion,  87,  96 
of  pressure  in  pipes,  124 

volumetric  efficiency  by  heat,  106 

Manifold  areas,  26 

deposition  in,  22 

frost  covered,  105 

intake,  the,  20 

lengths,  27 

velocities  in,  22 
Manometer  pressures,  121 
Marsh  gas  in  the  exhaust,  91 

Maximum  power  and  maximum  thermal  efficiency,  92 
Measuring  air  flow  by  orifice  in  thin  plate,  76 

flow  by  Venturi  meter,  80 
Mechanical  defects,  location  of,  47 


INDEX  135 


Mercury,  weight  of,  121,  124 

Method  of  gas  analysis,  98 

Miles  per  hour  to  feet  per  second,  122 

Mixing  valve,  the,  4 

Mixture,  constant,  17,  93 

effect  of  proportions  of  the,  105 

variable,  16 
Multiple  jet  carbureter,  the,  8 

Needle  for  fuel  regulation,  9,  10 

fuel  regulation,  inaccuracies  of,  9,  11 

Orifice  diameters,  selection  of,  79 

in  thin  plate,  76 
Orsat  apparatus,  101 
Oxygen,  presence  of,  in  the  exhaust,  97 

Partial  evaporation,  effect  of,  107 
Perfect  gas  law,  106. 
Performance  tests,  52,  93 
Performances,  comparing,  39,  48,  73,  93 
Petroleum  products,  combustion  reactions  of,  84 
composition  of,  84 
latent  and  specific  heats  of,  104 
Physical  conditions  of  carburetion,  103 
Poor  mixtures,  effect  of,  96 
Power,  effect  of  altitude  on,  113 
Practical  testing  of  motor-vehicles,  52 
Pressure,  absolute,  to  find  by  vacuum  gauge,  112 
atmospheric,  110 
drop  in,  by  velocity,  120 
effect  of,  on  the  auxiliary  valve,  111 
in  pounds  from  head  in  feet,  120 
volume  and  density  of  air,  127 
Priming  device,  necessity  for,  109,  117 
Product  of  the  carbureter,  character  of,  20 

Reactions  during  combustion,  84 

Reduction  of  temperature  by  evaporation,  105 

Relative  volumes  of  the  exhaust,  84,  95 

of  liquid  fuel  and  air,  9 
Report  of  tests,  form  for,  61 
Resistance  of  bends,  31 
Retardation,  44 
Rich  mixture  for  acceleration,  95 

effect  of,  95 

for  starting,  109 
Riedler's  results,  72 


136  INDEX 

Road  testing,  39,  52 

Rolling  resistance,  45,  54,  58 

Royal  Automobile  Club  standard  mixture,  92 

Rubber  diaphragm,  use  of  in  air-measuring  apparatus,  79,  81 

Sampling  for  gas  analysis,  98 
Simple  carbureter,  the,  4 
Specific  gravity  by  Baume  hydrometer,  126 
of  fuel,  effect  of  temperature  on,  3 

heat,  103 

of  petroleum  products,  104 
Speed,  check  on,  by  fuel  measurements,  66 

measurement  of,  57 

range,  64 
Starting,  conditions  necessary  for  easy,  25,  109 

hard,  causes  of,  24,  108 

rich  mixture  for,  109 
Summary  of  manifold  conditions,  33 

types  of  carbureters,  19 
Surging,  23 

Table  I.— Coefficients  of  discharge,  78 

Table  II. — Latent  and  specific  heats  of  petroleum  products,  104 

Table  III. — Compression  efficiency  at  altitudes,  119 

Table  IV.— Weight  of  gases,  125 

Table  V. — Baume  hydrometer  and  specific  gravities,  126 

Table  VI. — Volume,  pressure,  and  density  of  air,  127 

Tachometer,  use  of,  57 

Tapered  needle,  9,  10 

inaccuracy  of,  9,  11 

Temperature  correction  for  specific  gravity  of  gasoline,  125 
effect  of,  on  carburetion,  2,  3 
of,  on  flow  of  fuel,  3,  109 

of,  on  volume,  pressure,  and  density  of  air,  127 
reduction  by  evaporation,  105 
rise  from  given  B.  T.  U.,  126 
Testing  by  gas  analysis,  examples  of,  91 
motor-vehicles,  52 
on  the  block,  35 

the  block,  inadequacy  of,  39 
the  road,  39,  52 
with  fixed  load,  36 
Theory  of  carburetion,  1 
Thermal  efficiency,  determination  of,  49 
from  incomplete  combustion,  87 
in  relation  to  maximum  power,  92 
Time  element  in  explosion,  93 
Traction  drums,  55 


INDEX  137 


Transmission  friction,  determination  of,  46 
Types  of  carbureters,  summary,  19 

manifolds,  28 

Vacuum  gauge,  to  find  absolute  pressures  by,  119 
in  inches  of  water,  formula  for,  15 

relation  to  velocity,  15 
Valve,  weighted  air,  10 
Vaporization,  effect  of  altitude  on,  111 
partial,  effect  of,  107 
within  the  cylinder,  108 
Variable  fuel  orifice,  9 

mixtures,  16 

Velocity,  compensation  by,  13 
desirable  conditions  of,  115 
effect  of,  on  pressure,  120 
in  feet  per  second  to  inches  of  water,  123 
in  the  manifold,  21 
of  air,  1 

fuel,  2 

relation  of,  to  inducing  vacuum,  15 
the  only  constant,  17 
volumetric  loss  by,  22 

Venturi  meter,  application  to  carbureter  measurements,  81 
barometric  and  temperature  correction  of,  81 
calibration,  81 
principle  of  the,  80 
Vibratory  extinction  of  flame,  97 
Viscosity  of  gasoline,  109 

Volume  of  exhaust  from  various  air/gas  ratios,  95 
pressure  and  density  of  air,  127 
ratios  from  weight  ratios,  123 
Volumes,  relative  of  liquid  fuel  and  air,  9 
Volumetric  efficiency,  18,  124 
loss  of,  by  heat,  106 
loss  of,  by  velocity,  22 

Water,  weight  of,  121,  124 
Weight  of  gases,  125 

manometer  columns,  121 
ratios,  to  reduce  to  volume  ratios,  123 
Wimperis  accelerometer,  41 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 

Los  Angeles 
This  book  is  DUE  on  the  last  date  stamped  below. 


MAR      1974 
1874 


Form  L9-Series  444 


•     •       'II       II 

3  1158  00804  fi 


